Dragon Age Origins - Crazy amount of game time for me
Fallout 3 - My ex's brother described this as a game not meant to be beaten
Borderlands - Just plain fun
Call of Duty MWF 2 - I must begrudgingly admit, multiplayer is smooth and fun, 12 yr old asians kicking my ass isn't so much, but the game is overall.
Civilization Revolution - The ability to play a game of civilization in under 4 hours is huuuge!
Wii Fit - Drunken fun, and occasionally a fitness tool.
Edit - Forgot to add Pokemon. Yes I realize I am in my mid-20's, and that pokemon is kind of (really) childish, and the associated merch is outstandingly aggravating, but the desire to catch and evolve every pokemon across the game systems is crazy. I think I logged over 100 hrs on my first playthrough of red back in the day. Now when I go to the field on exercise, in my downtime I slog through an hour or two of Pokemon Diamond. Crazy good timesink, portable, with the ability to quicksave at any time, for like 40-60$. The games themselves are totally worth the money.