Beware,a long list of games beyond!
Cave Story- Fun run and gun sidescrolling platformer.Seriously awesome
Iji- Again,sidescrolling platformer gunning action.Also has a intriguing storyline(imo)
Battle for Wesnoth-Opensource turn based strat game.Pretty decent community, active multiplayer
UFO:Alien Invasion-Inspired by the x-com games,it's a mix of base management and turn based strat
Wolfenstein:Enemy Territory-Somewhat old by fun shooter.However,it uses punkbuster,so be warned.
Uhh,Gang Garrison 2-8 bit version of team fortress 2, rather impressive.
La-Mulana - Basically indiana jones in green,going through a big cave,finding stuff,whipping things.
Spelunky - It's like if la-mulana and nethack had a baby. Exploring the cave,with randomly generated levels,and 1 life.
Nethack - Incredibly in depth roguelike, focusing more on strats and clever item use, then grinding.
Dwarf fortress - Kinda like a ascii version of the sims.Except with dwarfs.
Even more games
Bunny must die- It's a japanese platforming sidescroller.However,there is a translation patch for it.
Umm,Kingdom of loathing - A pop culture filled online game with a amazing community.Also stickfigure art.
Nexus War - Web based mmo with a good community. You basically start as a mortal,and work your way up to become a neutral,angel,or demon. Really extensive.
Nanaca crash - Flash game based of a eroge, you basically smash into a guy with a bike,and he goes flying.Make him fly as far as possible.Yeah.
I think that's it,that i know of.