Advanced Mario Brothers
Wolfenstein: Enemy Territory is a Multiplayer Online First Person Shooter which sports some of the best graphics for a freeware first person shooter.
Eternal Daughter surpasses most RPG Nintendo games from the last decade in every single way.
Another excellent game from Freelunch Design (Johan Peitz; Creator of such amazing classics as Icy Tower)
Being an open source game, Well of Souls allows you to modify the game in any way you desire
injure someone, badly, in the ultimate stress reliever
The best platformer I have ever played
A fairly well made DOS based Monopoly game.
A classic adventure game of a peasant who wants to prove he is a hero
Play as Tux the penguin in this Super Mario clone.