Also I've heard good things about The Stanley Parable, which is supposed to be a Half Life 2 mod about video game narrative that's also super funny. But I think it's only available through Desura and I got very confused trying to download it (both the game and Desura)
EDIT: Turns out you don't need Desura to download it, but if you're playing on a Mac, you need Half Life 2 to play the mod
Anyway, I'd have to go with Marfitude being my favorite freeware game. It's like amplitude, but it supports .MOD files and automatically converts them to marfitude format.
unless he's talking about Enemy Territory. the free Return to castle wolfenstein multiplayer game.
which was pretty damn great
available here: http://enemy-territory.4players.de/news.php don't know if a lot of people still play it though
Check out anything by StabYourself, he always has funny games based off of existing classic games. There's Not Tetris (Tetris with physics) and Mari0 (Mario meets Portal), and he's coming out with something called Not Pacman