A Nights Haunting: Source
Action: Source
Age of Chivalry
Alien Swarm
Alien Swarm: Reactive Drop
Amalgam
AMYGDALA: Prelude
Aperture: Salt Mines
Aperture Tag: The Paint Gun Testing Initiative
Apex Legends
Anarchy Arcade
Arctic Adventures: Episodes
Battle Grounds III
Black Mesa
Blade Symphony
Bloody Good Time
BrainBread 2
Codename Cure
CONSORTIUM
Contagion
Counter Strike: Global Offensive
Counter Strike: Source
D.I.P.R.I.P. Warm Up
Dark Messiah of Might & Magic
Day of Infamy
Day of Defeat: Source
Dear Esther
Deeper Down
Destroyed Aperture
Dino D-Day
Divinia Chronicles: Relics of Gan-Ti
Double Action: Boogaloo
Dystopia
E.Y.E.: Divine Cybermancy
Empires
Entropy: Zero
Entropy: Zero 2
Estranged: Act I
Eternal Silence
Evacuation
Fast Detect
Fernandez Big Mystery
Firefight Reloaded
Fistful of Frags
Fortress Forever
From Earth
The Forgotten Ones
G String
Garry's Mod
Genry's Parables: Scientist Story
GROWTH
Half-Life: Source
Half-Life Deathmatch: Source
Half-Life 2
Half-Life 2: Episode One
Half-Life 2: Episode Two
Half-Life 2: Deathmatch
Half-Life 2: DownFall
Half-Life 2: Genry's Great Escape From City 13
Half-Life 2: Lost Coast
Half-Life 2: Update
Half-Life 2: VR Mod
Half-Life 2: VR Mod - Episode One
Half-Life 2: VR Mod - Episode Two
Half-Life 2: Year Long Alarm
Half-Life 2 RTX
HEAT REBORN
Hunt Down The Freeman
Infestus
INFRA
Inhuman
Insurgency
Insurgency: Modern Infantry Combat
IOSoccer
Jabroni Brawl: Episode 3
JBMod
KAYF-LIFE: Complete Edition
Kreedz Climbing
Lambda Fortress
Lambda Wars
Left 4 Dead
Left 4 Dead 2
Logistique Act. 1
Lost Squad
Military Conflict: Vietnam
MINERVA: Metastasis
Missing Information
Modular Combat
Momentum Mod

NEOTOKYO°
Nightmare House: The Original Mod
No More Room in Hell
Nuclear Dawn
Obsidian Conflict
Open Fortress
Operation Black Mesa
PASS Fortress
Pirates, Vikings, & Knights II
Portal
Portal 2
Portal BAGLEY
Portal 2 Sixense Perceptual Pack
Portal 2: Community Edition
Portal 2: Confinement
Portal: Alive & Kicking
Portal: Ambition
Portal: Intrusion
Portal: Lost in Aperture
Portal: Prelude RTX
Portal: Revolution
Portal Reloaded
Portal: Singularity Collapse
Portal Stories: Forbidden Testing Tracks
Portal Stories: Mel
Portal: System Failure
Portal Versus
Portal with RTX
Postal 3
Prospekt
PUNT
Quantum Paradox
Requiem Of Science
Resistance Element
Revelations 2012
Rexaura
Ruby: Tunnel Vision
The Ship
Silent Escape: Induction
SiN Episodes: Emergence
Smashball
Snowdrop Escape
SourceWorld
Swelter
Synergy
Team Fortress 2
Team Fortress 2 Classic
Team Fortress 2 Legacy
Team Fortress 2: Reborn
The Beginner's Guide
The Burton Equation
The Cleaning Game
The Stanley Parable
The Trap 2: Mindlock
Thinking With Time Machine
Titanfall 2
Transmissions: Element 120
Treason
Vampire: The Masquerade - Bloodlines
Vindictus
Wilson Chronicles
Zeno Clash
Zombie Master
Zombie Panic! Source

Alien Swarm FP ✔
Antartic Sciences
Aperture Science Laboratories: A Portal 2 Mod
Arctic Adventures: Episodes ✔
Blue Portals: Anniversary Edition ✔
Classic Offensive ✔
Crossroads Episode I ✔
CUBE ✔
Derayenic ✔
Destroyed Aperture
Half-Life 2: Awakening ✔
Half-Life 2: Capture the Flag ✔
Hypovolemia ✔
INSIGHT ✔
Joutomaa ✔
Killing, My Friend ✔
Last Year - 5v1 Survival Horror
Lost Story: The Last Days of Earth ✔
Missing Information ✔
Operation Black Mesa ✔
Portal: Alive & Kicking ✔
Portal: Outside Influence ✔
Portal Remake Mod ✔
Portal 2: Abyss ✔
Portal 2: Desolation ✔
Portal 2: Timex ✔
PUNT ✔
Reduction: The Alien Swarm Campaign ✔
Resurgence
Silent Escape ✔
Spherical Nightmares ✔
The Revenge ✔
The Trap 2: Mindlock ✔
Underhell ✔
Vanishing

✔ = Greenlit

Codename Gordon - Originally created to hype up the upcoming Half-Life 2. Was removed due to the in-game website defaulting to a porn site. Can still be installed using:
Half-Life: A Place in the West
Nightmare House: Reimagined - Remake of Nightmare House: The Original Mod on Unreal Engine 5.
Project Borealis: Prologue - An Unreal Engine 5 game that plays exactly like a Source game attempting to bring the Half-Life 2 episodic games to it's conclusion.
Project Three
Source Filmmaker
Stop The F

Dogfights
I, Predator
Ice Road Truckers
Kuma\War
Kuma\War 2
Kuma\War Classics
Kuma\War WWII\Vietnam
Mobsters
National Blood Sport
Street Soccer
The DinoHunters
The Kill Point Game
WWII Experience

Abatron - Originally on Source Engine when project was started on Greenlight. Has since moved on to Unreal Engine 4. Source version is no longer available.
Bear Party: Adventure - Unknown, removed without reason.
Perhaps still accessible using:
Conversion - Unknown.
Dear Esther: Landmark Edition - Switched to Unity, Source Engine version removed from Steam.
Dota 2 - Switched to Source 2.
Estranged: Act II - Switched to Unreal Engine 4 prior to release.
Faceless - Switched to Unreal Engine 4 prior to release.
Firearms: Source[steamdb.info] - Released prior to games having community hubs but can still amazingly be downloaded and played without issue. Even has achievements. Thanks Mr. Casual!
Haunted Memories - Switched to Unity.
Haunted Memories Episode 2: Welcome Home - Switched to Unity.
Lost Story: The Last Days of Earth Demo - Unreleased demo for unreleased game of the same name. An issue on Valve's side caused the page to appear before the official release.
Natural Selection 2 - Originally was built on Source Engine during development but was later moved over to the in-house Spark Engine.
NS2: Combat - Originally was built on Source Engine during development but was later moved over to the in-house Spark Engine.
Portal: Crossover - Unknown.
SiN Episodes: Emergence (DE) - License expired. Likely to return.
SourceForts - Unexplained. Page and App ID completely removed from Steam.
Tactical Intervention - Multiple promises to bring the game back, including a Kickstarter for a new paid for version of the game were made but these eventually led to nothing.
Thunder's Leaves - Unreleased/Publisher banned. Moved to new app id.
Thunder's Leaves - Banned again. Moved to new app id.
Thunder's Leaves - Banned for a 3rd time.
Time Ramesside - Switched to Unreal Engine 4 engine, was Source under the name A New Reckoning.
Titanfall - Persistent DDOS attacks on official servers. Game removed from both Steam and Origin.

Counter-Strike Online 2*³
Deep 6*
Flipside
Half-Life 2: Survivor*³
Hybrid*²
Korsakovia
Left 4 Dead: Survivors*³
Portal: Still Alive*²
The Crossing*
They Hunger: Lost Souls*
Twilight War: After the Fall*

* - Unreleased.
*² - Only available on Xbox 360.
*³ - Only available in Asia (Arcade)

Anything missing? Let me know. I'll add it. :)

List of GoldSrc Engine games
List of Source 2 Engine games
GoldSrc/Source/Source 2 Tracking Curator