I <3 Spore
Spore is the newest game coming from game genius Will Wright. Don't know who Will Wright is? Well, he made these obscure little games like Sim City, The Sims, and Civilization. ( See his history here )
Spore looks to be one of those games you get so soaked up in that each time you fire it up, before you know it, its 5am and you don't know whats happened.
I can't really do it justice with my words so go check out this 35 minute presentation on Spore. Coming in Fall 2007 (hopefully) this could dethrone World of Warcraft as my timesink of choice.