pordondemeda:

For ATI/AMD (Radeon) graphic card users: install - storefront
- Half-Life 2: Deathmatch [5]
- Half-Life 2: Lost Coast

For Nvidia (GeForce) graphic card users: install - storefront
- Half-Life 2: Deathmatch [5]
- Half-Life 2: Lost Coast
- Portal: First Slice (demo install - storefront)

For all Steam users:
Alien Swarm: install - storefront
America’s Army 3: install - storefront
ArmA II Free [1]: install - storefront
BattleForge [1][4][6]: install
Between [1][4]: install - storefront
Bloodline Champions [6]: install - storefront
Champions Online: Free for All [6]: install - storefront
Codename Gordon: install
COIL [1]: install - storefront
CrimeCraft: Bleedout [6]: install - storefront
DC Universe Online [6]: install - storefront
Global Agenda: Free Agent [6]: install - storefront
Iron Grip: Marauders [6]: install - storefront
Mightier [1]: install - storefront
Moonbase Alpha: install - storefront
Peggle Extreme: install - storefront
Puzzle Pirates [6]: install - storefront
Rise of Immortals [6]: install - storefront
Sam & Max Episode 104: Abe Lincoln Must Die!: install
Spacewar: install
Spiral Knights [6][7]: install - storefront
Team Fortress 2 [6]: install - storefront
The Cat and the Coup: install - storefront
TrackMania Nations Forever [2]: install - storefront

For regional Steam users: (aggressive IP block or content access restriction can be applied)
Alliance of Valiant Arms [6][R1]: install - storefront
Forsaken World [6][R2]: install - storefront
Rusty Hearts [6][R3]: install - storefront
War Inc. Battlezone [6][R3]: install - storefront

Free addons and resources for already purchased games:
Dungeon Defenders (The Great Turkey Hunt! Mission & Costumes DLC) [8]: install - checkout - storefront
Dungeon Defenders (Dungeon Defenders Development Kit DLC) [8]: install - checkout - storefront
Dungeon Defenders (Dungeon Defenders Capture the Flag Pre-Alpha Pass DLC) [8]: install - checkout - storefront
Half-Life (high definition models): install - uninstall - comparison - screenshot
Post Apocalyptic Mayhem (Death Area 8 DLC) [8]: install - checkout - storefront
RACE 07 (Crowne Plaza Raceway track): install - storefront
Railworks 2 (Billionaire Benefactor Scenario DLC) [8]: install - checkout - storefront
Railworks 2 (Keeping Up with the Diesels Scenario DLC) [8]: install - checkout - storefront
Railworks 2 (Off the Cotswold Line Career System Scenario Pack DLC) [8]: install - checkout - storefront
Railworks 2 (Port Road Expansion Pack DLC) [8]: install - checkout - storefront
Railworks 2 (UK Buildings Model Pack DLC) [8]: install - checkout - storefront

Steam delivered MODs:
Age of Chivalry [9]: install - storefront
D.I.P.R.I.P. Warm Up [9]: install - storefront
Darkest Hour (requires Red Orchestra): install - storefront
Defence Alliance 2 (requires Killing Floor): install - storefront
Dystopia [9]: install - storefront
Empires Mod [9]: install - storefront
Eternal Silence [9]: install - storefront
INSURGENCY [9]: install - storefront
Mare Nostrum (requires Red Orchestra): install - storefront
Pirates, Vikings, and Knights II [9]: install - storefront
Smashball [9]: install - storefront
Synergy (requires Half-Life 2): install - storefront
Zombie Panic! Source [9]: install - storefront

Related stuffs:
BioShock: Breaking the Mold [Art]: download - storefront
Company of Heroes: All Heroes Rise OST [OST]: download - storefront
Company of Heroes: Songs From the Front OST [OST]: download - storefront
Garshasp: The Monster Slayer Art Book [Art]: download - storefront
GREED: Black Border OST [OST]: download - storefront
Warhammer 40,000: Dawn of War II OST [OST]: download - storefront
Zombie Movie [Mac/Win][Mov]: install - storefront

Honorable mentions:
Portal 2 Soundtrack: Volume 1 [OST]: download - website
Portal 2 Soundtrack: Volume 2 [OST]: download - website
Portal 2 Soundtrack: Volume 3 [OST]: download - website

Storefront links:
Demos - Mods - Videos and trailers

Useful topics:
List of Steam Games with DRM - Unpatched Games
Free Steam games list - Free games list and 3rd party mods
Steam Deals and Price Reductions! - The SPUF Off Topic theme song

== notes ==
[1] mistakenly listed as a demo on storefront
[2] already included on TrackMania United
[3] deployed and deprected, but continues working
[4] mistakenly named as a demo on “My games” list
[5] does not include Source SDK tool (required to run some MODs)
[6] offered by Play4Free license / Free 2 Play / micropayment
[7] storefront not available worldwide, can be installed without restriction
[8] checkout required to include item on account
[9] mistakenly listed as source game dependent on storefront to get Source SDK (now free for all) 
[R1] Available only for AND, AUT, BEL, BGR, CAN, CHE, HRV, CYP, CZE, DEU, DNK, ESP, FRA, GBR, GRC, HUN, ISL, IRL, ITA, LUX, MLT, MNE, NLD, NOR, POL, PRT, ROM, SRB, SVK, SVN, SWE, USA
[R2] Not available for AUS, CHN, HKG, IND, JPN, KOR, MAC, MYS, PHL, PRK, RUS, TWN, VNM
[R3] Not available for RUS
[R4] Available only for USA

[Mac/Win] operating system compatibility*
[Art/OST/Mov] content type (Artbook/Original Soundtrack/Movie)**

(Fuente: correplatanito)