Half-Life 2 Garry's mod
Full user review
"How to install Garry's Mod 9 for free:"
Free and fun! Can't beat that. Many free addons at freesteam.org.
No longer officially supported.
1. Install Steam & make account: store.steampowered.com/about/Also install Source SDK Base 2007. You may need it for some multiplayer servers.
2. In Steam go to View>Tools & install Source SDK Base 2006.
3. Go to Steam>Exit to exit Steam. Install Gmod 9.04
4. Open Steam\steamapps\sourcemods\gmod9\gameinfo.txt & change SteamAppId 220 to 215 & save. Load Steam, go to View>Games List and click Gmod to play.
5. Many free Gmod addons at freesteam.org.
More free Steam games here: forums.steampowered.com/forums/showthread.php?t=851573
Updated on Oct 2, 2011