The Older Gamers Forums - Steam Freebies.

I just found this thread over on the official steam forums that details free games and mods that are available on Steam :):-
Free Steam games list - Steam Users' Forums

Quote:

Free Steam games list

All steam:// links will only work if you already have Steam installed on your PC.
Your internet browser can ask for confirmation [IE][Firefox] to run these links.

For ATI/AMD (Radeon) graphic card users:
Link: steam://hardwarepromo/305
- Half-Life 2: Deathmatch [6]
- Half-Life 2: Lost Coast

For Nvidia (GeForce) graphic card users:
Link: steam://hardwarepromo/609
- Half-Life 2: Deathmatch [6]
- Half-Life 2: Lost Coast
- Peggle Extreme (free)
- Portal: First Slice (demo)

For all Steam users:
America's Army 3 [7]: steam://install/13140
Between [1][4]: steam://install/29140
Codename Gordon: steam://install/92
COIL [1]: steam://install/31500
Mightier [1]: steam://install/29150
Peggle Extreme: steam://install/3483
Sam & Max: Episode 4: steam://install/8230
TrackMania Nations Forever [2]: steam://install/11020

Free addons and resources for already purchased games:
Counter-Strike (Counter-Strike Steamworks Beta access) [3]: steam://install/150
Half-Life (high definition models - comparison): steam://installaddon/halflifehd (uninstall)
RACE 07 (Crowne Plaza Raceway track) [5]: steam://install/8670

Official MODs:
Age of Chivalry (requires Source SDK): steam://install/17510
D.I.P.R.I.P. Warm Up (requires Source SDK): steam://install/17530
Darkest Hour (requires Red Orchestra): steam://install/1280
Dystopia (requires Source SDK): steam://install/17580
Eternal Silence (requires Source SDK): steam://install/17550
INSURGENCY (requires Source SDK): steam://install/17700
Mare Nostrum (requires Red Orchestra): steam://install/1230
Smashball (requires Source SDK) [8]: steam://install/17730
Synergy (requires Half-Life 2): steam://install/17520
Zombie Panic! Source (requires Source SDK): steam://install/17500