• February2021 Update: Reworked the guide, it should be up to date again.
----------

Exclusive DLC, no longer distributed
• Career Criminal Edition
• Completely Overkill Pack
• Lootbag DLC
• Scarface Character Pack
• Alienware Alpha Mauler Weapon
• Community Safe Reward 1
• Community Safe Reward 2
• Community Safe Reward 3
• Community Safe Reward 4
• Community Safe Reward 5
• Community Safe Reward 6
• Community Safe Reward 7
• Community Safe Reward 8
• Community Safe Reward 9
• Community Armor Safe Reward 1

Exclusive DLC not available via the Steam Store
• Alienware Alpha Mask Pack
• Humble Bundle Mask Pack
• Humble Bundle Mask Pack 2
• Humble Bundle Mask Pack 3
• Humble Bundle Mask Pack 4
• Humble Bundle Mask Pack 5
• Jack Mask Pack
• Queen Mask Pack
• King Mask Pack
• Joker Mask Pack
• The Almir

Exclusive DLC still readily available for free
• Paydaycon 2015
• Paydaycon 2016
• Sydney Mega Mask
• Housewarming Party
• Pen DLC
• Fedora
• Simon mask

Exclusive DLC still readily available via purchase of merchandise
• Dozer Bobblehead Mask

Content acquired via joining other games' communities
• Dead By Daylight
• The Solus Project
• RAID: World War 2

Content acquired via owning other games
• Hotline Miami
• Hotline Miami 2
• Hotline Miami 2, Deluxe Edition
• Dead By Daylight, Deluxe Edition
• Speedrunners
• Payday: The Heist's Wolf Pack DLC (PD2:WolfPack is included in "PD2 Legacy Collection")
• Antisphere

Career Criminal Edition
This was only available as a preorder option, and thus became unavailable once the game released in August 2013.
--Practically unobtainable, codes are no longer being produced for this DLC; only possible to acquire via trading with those who have extras; Steam keys may still exist but should be considered extremely rare.
Contents: Lootbag DLC, 2 Beta keys, PD2 OST (not the Soundtrack DLC, though the CCE included all the same content as the Soundtrack DLC, including the Poetry Jam masks), a discount on CrimeNet purchases (later to be included into the base game), Bain's Guide PDF, and Blueprints of various heists (not in-game) that the owner could print out. Blueprints were added to the Bain's Guide PDF file.



Lootbag DLC
This was the standard preorder bonus. Preordering any version of the game (base or CCE) gave you the Lootbag DLC. As such, it became unavailable once the game released in August 2013.
--Very difficult to obtain, codes are no longer being produced for this DLC; only acquired via trading or from large, Overkill-sponsored contests on the forums/reddit. (very hard to obtain)
Contents: Unique Red Dot sight weapon mod, Skull mask, I ♥ Overkill pattern, Black/Red colors, small bundle of in-game cash.



Completely Overkill Pack
This was a time-exclusive DLC that was only available for purchase during Hype Train in 2015 for $20, and was limited to a quantity of 50,000 copies.
--Extremely difficult to obtain, only acquired via trading with those who have extra copies; trades go for hundreds of dollars (USD); no Steam keys exist for COP, only tradeable inventory copies.
Contents: 14 MEGA masks, a Completely Overkill Safe and accompanying Drill to open it (Safe contained one random COP weapon skin).



Scarface Character Pack
This DLC was released on Dec 15, 2016.
On Sep 22, 2020, the Overkill developers posted an announcement that the sale of this DLC (individually and within the Legacy Bundle) would discontinue after Oct 1, 2020, almost assuredly due to an expiring license agreement.
Contents: the Scarface character, a perk deck, two masks, a primary firearm, a melee, and a "throwable" only used by the previously mentioned perk deck.



Alienware Alpha Mauler Weapon
This was previously a free code acquired on AlienwareArena's website, but Alienware shut down the page in spring of 2017. The offer was then moved to their Alienware Arena app via the following steps:
- Downloading the AlienwareArena mobile app
- Creating or logging into an AlienwareArena account
- Navigating to the Giveaways tab
- Scrolling down to the bottom of the long list of offers, to the Payday 2 Alienware Alpha Mauler offer
Mauler codes were confirmed to still be available as late as March 2019, but in June of 2020 it was learned that the app had stopped functioning correctly at some point, and the Alienware Mauler codes were no longer obtainable. The app proved difficult to navigate, and all offers present on the app refused to complete successfully. Efforts were made to locate a web-based giveaway page for the Mauler, but none were found; unless someone can find a way to get Giveaways to work on the AwA app, the Mauler codes seem lost for good.

Note: The Alienware Mauler has the exact same stats as the Ding Dong Breaching Tool, this is effectively just a reskin with a sound effect when you swing it.



Community Safe Reward 1
Community Safe Reward 2
Community Safe Reward 3
Community Safe Reward 4
Community Safe Reward 5
Community Safe Reward 6
Community Safe Reward 7
Community Safe Reward 8
Community Safe Reward 9
A DLC code that is only given out to members of the community who create and post weapon skins to the Workshop and have their skin chosen to be featured in a particular Community Safe. Each of these codes give the user one Mint Condition copy of every weapon skin from the Safe in which their skin is featured. Weapon skins granted in this fashion are tradeable/marketable on the Steam Market.


Community Armor Safe Reward 1
A DLC code that, like the weapon-variant Community Safes, is only given to those who create armor skins on the Workshop and have their skin chosen to be featured in a Community Armor Safe. Those chosen get one copy of every armor skin (tradeable/marketable) from the Safe in which their skin is featured.

Alienware Alpha Mask Pack
This was a cross-promotion starting in November 2014, which was provided with the purchase of an Alienware Alpha Steam Machine. However, by May 2016, Alienware stopped providing PD2 Mask Pack codes alongside Alpha purchases.
--Moderately to Very difficult to obtain, afaik codes are no longer being produced for this DLC; generally only acquired via trading or from large, Overkill-sponsored contests on the forums/reddit.
Contents: 2 masks.




Humble Bundle Mask Pack
The first PD2 HB Mask Pack, available in June 2014 in HB's E3 bundle. The pack was offered again in a bundle in January 2017.
--Uncommonly/Rarely available in giveaways on Forum/Discord/Twitch/Twitter, or during contests.
Contents: 2 masks.



Humble Bundle Mask Pack #2
The second PD2 HB Mask Pack, available in October 2014 in HB's Halloween bundle, but was also obtainable at the time by simply entering your email (no purchase necessary). This pack was offered again in a bundle in January 2017.
--Uncommonly/Rarely available in giveaways on Forum/Discord/Twitch/Twitter, or during contests.
Contents: 2 masks.



Humble Bundle Mask Pack #3
Along with HB#4, this was available in HB's 2015 E3 bundle. HB#3 was offered again in a bundle in January 2017.
--Uncommonly/Rarely available in giveaways on Forum/Discord/Twitch/Twitter, or during contests.
Contents: 2 masks.



Humble Bundle Mask Pack #4
Along with HB#3, this was available in HB's 2015 E3 bundle. Unlike HB#1, #2, #3, and #5, this mask pack was not available in January 2017.
--Uncommonly/Rarely available in giveaways on Forum/Discord/Twitch/Twitter, or during contests.
Contents: 2 masks.



Humble Bundle Mask Pack #5
This was available in HB's 2016 E3 bundle, and was offered again in January 2017.
--Uncommonly/Rarely available in giveaways on Forum/Discord/Twitch/Twitter, or during contests.
Contents: 3 masks.



Jack Mask Pack
Queen Mask Pack
King Mask Pack
Joker Mask Pack
These four masks were given out individually to those who attended 2015 E3. Overkill handed out physical cards, one for each mask, which had a code for their respective mask on the back.
--Uncommonly/Rarely available in giveaways on Forum/Discord/Twitch/Twitter, or during contests.



The Almir
(As I've heard the story..)
Originally intended to be linked to physical merchandise, the items never made it from distributors to actual sale in retail locations. The goods, and thus the codes, were "acquired" by individuals in China from the local distributors and the mask codes began appearing online for private sale by individuals. To combat the scalping of codes, PD2 developers deactivated the codes tied to the merchandise, and have since generated and been giving codes to content creators to give out to viewers for free.
- The merchandise in question was Zag Toys Payday 2 Backpack Hangers, where each individual product was a blind bag. Almir hangers were the rare pull in a series of 8, and included with each physical Almir hanger was a code for the digital in-game mask.
--Uncommon/Rare, available in giveaways by "Overkill Partner" content creators.

Paydaycon 2015 Mask Pack
The 'King of Jesters' mask, this was given out to those who went to PAYDAYCON 2015 in Seattle, USA in August.
-Previously awarded as a prize in various giveaways and contests, now available here[www.overkillsoftware.com].



Paydaycon 2016 Mask Pack
The MEGA Clover Mask, this was given out to those who went to PAYDAYCON 2016 in Melbourne, Australia in November.
--Previously available as a prize in various giveaways and contests, now available here[www.overkillsoftware.com].



Sydney Mega Mask
A single mask, once given out to those who donated at least $10 to Georgia VanCuylenberg's (the voice of Sydney) charity.
-- Was later commonly distributed in various giveaways and contests, now available here[www.overkillsoftware.com].



Housewarming Party
Acquire free by visiting Overkill's Housewarming Party Hat code page[www.overkillsoftware.com].
(In times when Overkill's site is experiencing high traffic, the page might not load. Try using https instead -- link: https://www.overkillsoftware.com/housewarmingkeys/ -- or try accessing the site from a mobile device. If neither work, wait a couple days then try again.)



Pen DLC (Pencil Melee)
Acquire free by visiting Overkill's Pen DLC code page[www.overkillsoftware.com]. And no that wasn't a typo, it is called "Pen DLC" despite giving you a pencil melee.
(In times when Overkill's site is experiencing high traffic, the page might not load. Try using https instead -- link: https://www.overkillsoftware.com/kentokeys -- or try accessing the site from a mobile device. If neither work, wait a couple days then try again.)


Fedora DLC
Acquire free by visiting Overkill's Fedora DLC code page[www.overkillsoftware.com].
(In times when Overkill's site is experiencing high traffic, the page might not load. Try using https instead -- link: https://www.overkillsoftware.com/fedorakeys/ -- or try accessing the site from a mobile device. If neither work, wait a couple days then try again.)


Simon Mask
Acquire free by visiting Overkill's Crimefest 2017's DLC code page[www.overkillsoftware.com].
(In times when Overkill's site is experiencing high traffic, the page might not load. Try using https instead -- link: https://www.overkillsoftware.com/lockekeys/ -- or try accessing the site from a mobile device. If neither work, wait a couple days then try again.)


Weapon Color: Army Gray
Acquire free by visiting Overkill's Social Safe[www.overkillsoftware.com] webpage.

Dozer Bobblehead Mask
Purchase a Dozer Bobblehead and a code will ship with it in the box.
Purchase can be made at Overkill's online store[store.overkillsoftware.com].

Dead By Daylight
5 masks for joining/following the DBD community (go to the DBD store page and click the Follow button).



The Solus Project
1 mask for joining/following The Solus Project community (go to TSP store page and click the Follow button).



RAID: World War 2
1 melee weapon for joining/following the RAID: World War 2 community (go to the RAID store page and click the Follow button).

Hotline Miami
(4 masks, 4 materials, 4 patterns, 1 melee)



Hotline Miami 2, Standard or Deluxe Edition
For owning HM2, any version of it, you get 6 masks in PD2.



Hotline Miami 2, Deluxe Edition
For owning the Deluxe Edition you get the aforementioned 6 masks, plus the Jacket Character Pack which includes: 2 more masks (Richard Returns and Richard Begins), a melee, an SMG, and Jacket as a playable character.
(The Jacket Character Pack can be obtained via Steam Keys given out during various giveaways - additionally, due to the unavailability of Hotline Miami 2 in AUS, Australians can email Overkill with proof of AUS residency to obtain a key, but the response time via email is generally reported by AUS players to be very, very long.)



Dead By Daylight, Deluxe Edition
Comes with 2 more masks (in addition to the 5 community masks).
**Note that DBD does NOT OFFER an upgrade option to go from base game to Deluxe. If you buy the base game, in order to get PD2's masks you'll either have to purchase the Deluxe edition (and not get anything back for the base version you just overwrote), or refund the base game if able..



Speedrunners
Owning this game gives you 4 masks in PD2.



The Wolf Pack DLC (Payday: The Heist)
The PD2: Wolf Pack DLC -is- included in the Payday 2 Legacy Collection.
Owning the original Wolf Pack DLC from PD:TH gives PD2 players the newer "PAYDAY 2 Wolf Pack DLC" for free. The PD2 Wolf Pack DLC is also available as an individual purchase on the PD2 store, but owning this PD:TH DLC will provide it for free.
http://steamproxy.net/steamstore/app/207815/PAYDAY_The_Heist_Wolfpack_DLC/ http://steamproxy.net/steamstore/app/441600/PAYDAY_2_Wolf_Pack/

Antisphere
Owning this game gives you 6 masks in PD2.

The original MEGA Masks were unique due to the special cosmetic visual/particle effects on them that would persist through mask customization.

While Mega Masks were originally exclusive to the limited-time Completely Overkill Pack DLC, the use of visual effects has been made more easily accessible to all players since that time. The original COP masks are still only available to the owners of said DLC, but there are other, newer masks that are not nearly as exclusive.

Below is a comparative shot of COP masks versus other exclusive MEGA masks, versus some examples* of MEGA Masks that are included in the base game / Ultimate Edition.
(*Some base-game MEGA masks not pictured below include Golden Chains, the One Down mask, the achievement-milestone masks, etc ... these are not exclusives)

I think that's all of it.
As far as I am aware, the only way to get "Career Criminal Edition" or "Completely Overkill Pack" DLCs nowadays is to find someone who still has a copy in their Steam inventory and trade for it, as codes do not exist for these. Codes for "Lootbag" and "Alienware Alpha Masks" are very hard to come by but show up in forum contests here and there.

Best of luck!