I think if you want something that will suck your life up, and you like third-person shooters and sandbox GTA-like games, you could NOT do better than to buy Just Cause 2. It has absolutely owned my life for the last week, and shows no signs of letting up. Its the first game I've bought at the full $50 price in literally 6-7 years, and I don't regret a penny of it. I feel like its already paid for itself in entertainment value, and I'm, what, like 11% complete with it? Awesome. Just awesome.

Also, I have no idea if you've ever played the Half-Life series, but if not, well worth your time. The individual games aren't too long, from what I understand, but if you buy 'em all, you'll have a lot of absolutely awesome gaming time on your hands. (I can only speak from experience for HL 1, which I'm...I dunno, maybe 1/3-1/2 of the way through, but its awesome, and the sequels only get better from what everyone says.)

if you like RTSes, Empire: Total War will also be a huge and entertaining life-sink. Not only is the "story mode" fun, there are so many different factions, ways to play, and win conditions you can set that the re-playability is huge. (I would advise staying away from any of the DLC, at least at first...the extra units really, really aren't worth your money unless you play the game a lot and become a fan of using certain factions--then buying the extra units for that faction might be worth it.)

Also on the RTS front, Civilization IV. Just...Civ IV.

If you haven't played Batman: Arkham Asylum has a relatively short (but incredibly fun) story mode. The after-game play, searching for all the little collectibles, unlockables and riddles will keep you busy, though (as will replaying it just for fun.)

GTA: San Andreas if you have a PS2 and haven't played it. SO much damn fun, and the 90s nostalgia makes it even more so. Failing that, if you don't have a PS2/already have SA, GTA IV is pretty good. Personally it wasn't really my cup of tea; the story didn't do it for me and neither did the missions or environment. The size of the map just seemed intimidating as opposed to fun to explore. Still, if you like GTA, its a great buy on Steam right now at $30. (I'd recommend Just Cause 2 over it all day long, but JC2 IS still $50 and, although you will get more than enough bang for your buck to justify the price, GTA IV also has a ton of content for its $30.)

Also, Portal. Its a very short game, but its challenge modes and achievements will keep you playing over and over again. As will the absolutely wonderful story. And you can't beat the game quality you get for the price.