Wolfenstein: Enemy Territory is a Multiplayer Online First Person Shooter which sports some of the best graphics for a freeware first person shooter.
Advanced Mario Brothers
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)
injure someone, badly, in the ultimate stress reliever
Being an open source game, Well of Souls allows you to modify the game in any way you desire
The best platformer I have ever played
A fairly well made DOS based Monopoly game.
Play as Tux the penguin in this Super Mario clone.
A classic adventure game of a peasant who wants to prove he is a hero