When I get around to using Steam here's some ones I'll hide:
Hacker Evolution Duality: Somebody got me this for Christmas once. I feel bad for never playing it, but it is really not my type of game. In fact, as far from it as can be.
Audiosurf: Not a bad game, but I am sick of it. It was the only game I had on Steam for something close to three months, and the appeal doesn't last long. Some fate for the game that convinced me to get Steam.
Star Wars Dark Forces 2: Jedi Knight: Bought it in a craze for classic shooters, then was utterly tired of it before I got halfway through. I think Mission 2 is longer than many full-sized games. And the lack of music makes it very lonely and creepy as well.
Jedi Knight: Jedi Outcast: Beat it once, no reason to look back really.
Sven Co-op: I just wanted to play Half Life's campaign cooperatively, but all the servers are for modded games with accounts and forums and clans and experience and I just wanted to shoot aliens okay
Combat Arms: Since it's free it might just disappear from my list when I uninstall it, but I keep getting tempted to play and realizing it's terrible all over again.
DOTA 2: I had some pretty bad experiences with League of Legends and getting actual IRL mad over a bloody video game, so I'm kind of soured to MOBAs. Also, it never installed properly and has been perpetually updating ever since.