Fallout 3 is so amazingly addicting that everytime I go near it I snap out of a stupor and realize its days later.
As for pure time-wasting fun, my absolute favorite game for that was/is Incredible Hulk: Ultimate Destruction for the last-gen of consoles. I would just boot it up and spend hours demolishing anything and everything I could find, and call in strike teams whenever I felt like a challenge by grabbing civilians, running up sky scrapers, putting them down gently, and watching as they would leap off in terror.
This is why I'm so excited for Prototype. It's got the same people making it, it's got all sorts of cool new ways to play like stealth (it's hard to imagine the Hulk being sneaky), badass new abilities, an even higher level of destruction than Hulk:UD (plus it has gore! Yay!), and massively improved visuals. As long as nothing catastrophic happens, I'm confident that Radical will succeed in bringing sexy back.