Fallout 3, which I've been working through for at least two years and might finish one day.
Age of Conan if I can be bothered to reboot into Windows to play it.

Edit: I forgot Saints Row 2, probably because I also have to reboot into Windows to play it.

Beneath a steel sky
New Vegas
F1 2010
Soon (When my friend puts my money on my paypal): PS:T

...and now I am done.

I have a bad habit of keeping multiple games going at once. I kind of chalk it up to not having a ton of time during the week to game so if I have a few that I can do in short bursts til the weekend, I'll be good. That I'm readily playing:

Call of Duty: Black Ops,
Mafia 1,
PST

All of those are on the PC.

New DLC for Fable 3 comes out tomorrow so I might be hopping back to my 360 soon to check that out.

trying out teenagent at the moment might consider buying Duke Nukem.

Baldur's Gate
Fallout

Plants vs. Zombies
Dead Rising 2
Alan Wake

and [url=http://www.gog.com/en/gamecard/freespace_2]FreeSpace2 (pimped up to OpenFreespace in 1600x1200 native on Linux).

SD GUNDAM Online
Left 4 Dead 2
Minecraft
Touhou 12.9 Great Fairy Wars

puzzle quest 2

Poker Night at the Inventory

This. :)

Actively :
Planescape Torment
Sam&Max season 2

Occasionnaly :
Darkstar One
King's Bounty
Neverwinter Nights (2-players LAN)

I might get Stalker : Call of Pripyat soon, I loved the first two episodes, but I'll need some more free time for this one.

As I finished Outcast yesterday, I started Runaway A Road Adventure and will most likely also start playing Puzzle Agent.

I just started playing Dungeons and Dragons Online this week. I wasn't impressed with it at first. But now I think I'm addicted. It's a breath of fresh air compared to Entropia.

I played Tidalis last night and I think it's a very good and well designed casual puzzle game. You can grab it today as part of the Steam's Indie Puzzle Pack (together with 4 more awesome puzzle games). Worth the $5 sale price IMO, heck, the Tidalis alone is worth more than that.