>Doom 1 and 2
>Quake 1-3 and Live (and maybe 4 too)
>CS 1.6 (and maybe Source)
>Half-Life and Mods (and maybe Half-Life 2 and certain mods)
>Sven Co-Op
>Team Fortress Classic
>Star Wars Battlefront 2
>Unreal Tournament 99 and 2K4
>Return to Castle Wolfenstein and Enemy Territory
>Red Faction
>Star Wars Jedi Knight 1 and 2 and Jedi Academy
>Halo Custom Edition and Halo 2 Project Cartographer
>Call of Duty 1, 2, and 4 (4 should be able to run on a toaster since it's pretty old, but your mileage may vary, I wouldn't recommend it if your computer's more than 10 years old)
>Team Fortress 2 Vintage (stay away from the current version of the game, as it's bloated as fuck and not toaster-friendly)
>Supreme Commander 1
>Total Annihilation
>Starcraft
>Command and Conquer series up until 3
>Warcraft II and III
>Diablo I and II
>GTA III, Vice City, and San Andreas
>World of Warcraft with a vanilla custom server
>Battlefield 1942, 2, and 2142
>Rainbow Six 3
>Splinter Cell Chaos Theory (beware that setup is a pain in the ass and requires use of software like Evolve if you want to play online)
>Giants: Citizen Kabuto
>Everquest (with Project 1999)
>Mechwarrior series (beware that setting these up is a pain in the ass too, though the communities are still reasonably-active last I checked)
>Descent 1 and 2
>Star Wars: X-Wing Alliance (not to be confused with X-Wing Vs. TIE Fighter, which is a shit game contrary to popular belief)
>Broforce (maybe)
>Borderlands 1 (maybe)
>Castle Crashers
>E.Y.E. Divine Cybermancy (maybe)
>Sins of a Solar Empire (maybe)
>Age of Empires 1 and 2, and Age of Mythology