Team Fortress 2.
And I think Steam has a whole section for free games, been a while since I've used it though.
Team Fortress 2 is a list of free full PC games.
Right.
But Steam does indeed have a
very little amount of free games.
The best choice would be to hook up with Google, because it can end you to a free game, or multiple lists of free games.
Google
"free" pc games download -piracy -torrent -buy -purchase and you will get a bunch of such lists, try variations too.
Variations:
pc games download -piracy -torrent -bigfishgames -buy -purchase -trial -demo
"pc games" download -piracy -torrent -buy -purchase -trial -demo
free full games list -buy -purchase
Also Wikipedia released an article that shows games that were previously commercial/shareware but were changed to freeware license (first result of last variation):
http://en.wikipedia.org/wiki/List_of_commercial_video_games_released_as_freeware