Witcher I/II
The Elder Scrolls Series
Jade Empire
Mass Effect Series
Vampire the Masquerade: (Redemption and Bloodlines)
Borderlands
Mount and Blade: Warband
Fallout 3 / Fallout: New Vegas
S.T.A.L.K.E.R.
If you want to get into more RPG and less Action:
Baldur's Gate
Neverwinter Knights
Divine Divinity
Diablo I/II, III in a year or so when it drops in price (not worth 60 bucks imo)
Dragon Age: Origins/Dragon Age 2
Fallout 1/2
Just to fire off some of the better ones I've played.
EDIT: In terms of time sink, Warband. is by far the most replayable of those titles. In addition to the enormous singleplayer aspect, there's a fairly sizable and dedicated multiplayer community. I've easily lost hundreds of hours to that game, and it has a fairly significant mod scene to make it even more soulsucking. It's a standalone expansion to the original M&B, but it's basically the same thing just made better. It's 20 bucks on Steam or through the Taleworlds website.
There's actually a new one coming out that looks pretty sweet, called War of the Roses. They were at GDC this year in a very Alpha state.
http://www.youtube.com/watch?v=EnMkNiU7HTw&feature=related
