I was keeping a list and I've downloaded a bunch but haven't got around to them. Here's the list (Sorry I lack links):
Action Games: Alien Swarm, Grand Theft Auto, Grand Theft Auto 2, Guxt, Mechwarrior 4, The Suffering
Adventure Games: Beneath a Steel Sky, Digital: A Love Story, Don't Take it Personally Babe This Just Ain't Your Story, King?s Quest I, King?s Quest II, King?s Quest III, Quest for Glory II
Puzzle Games: Peggle Extreme
Fighting Games: Vanguard Princess
FPS/TPS Games: Doom, Duke Nukem 3D, Marathon, Marathon 2, Marathon Infinity, Quake, Team Fortress 2, Tribes 2, Wolfenstein 3D
Platforming Games: Abuse, Blendimals, The Blob, Cave Story, Fragotto & Friends, N, Spelunky
Racing Games: Trackmania Nations
Rhythm Games: Frets on Fire, Osu!
Role-Playing Games: Akuji the Demon, Beautiful Escape: Dungeoneer, Clock of Atonement, The Elder Scrolls: Arena, The Elder Scrolls II: Daggerfall, Eternal Daughter, Ikachan, Nethack, Professor McLogic Saves The Day, Space Funeral, Ultima IV: Quest for the Avatar
Simulation Games: Cloud
Strategy Games: The Battle for Wesnoth, Dwarf Fortress, Star Contol 2
There's some classics on there, obscure indies, and a few games that have more recently been made free. It's not super up to date and I've barely played any of them. Man, I have a lot of stuff to play despite having no money...