YouTube
AStats [astats.astats.nl]
Recent games and 100%'s [astats.astats.nl]
Completionist [completionist.me]
Steamgifts [www.steamgifts.com]

AStats [astats.astats.nl] is the tracking site I use for Steam achievements. Offers vast data of statistics, guides, list of broken achievements, giveaways, playlist, etc.
IsThereAnyDeal [isthereanydeal.com] is where I keep my wishlist. Allows tracking of deals from multiple stores for games, bundles, vouchers, giveaways.
Idle Master Extended [github.com] helps with farming trading cards from games.

They're kind of pointless and hold no real value, so I can't give a valid reason for chasing them. At least I met a bunch of great folks thanks to them, so there's that.

American Truck Simulator - 53/75 (missing DLC)
Black Mesa - 25/50
Cubic - 17/43
*Dolphin Swim - 64/68 (*restricted)
Euro Truck Simulator 2 - 67/81 (missing DLC)
Games&Girls - 8/10
Helltaker - 8/10
*Inside The Computer - 2/4 (*restricted)
Kyle is Famous - 0/18
LOGistICAL: Earth - 707/2600
*Lucid Dream - 6/7 (*restricted)
Nomad - 26/27
PUBG: BATTLEGROUNDS - 29/37
*Project Speed - 0/4 (*restricted)
*Second Chance - 56/73 (*restricted)
*Sharp - 2/3 (*restricted)
Unturned - 46/63
*Welcome To... Chichester 2 : The Spy Of Chichester And The Eager Tourist Guide - 29/31 (*restricted)
*Welcome To... Chichester OVN : The Beach - 20/26 (*restricted)