Games that make you lose track of time.

So I've been playing Civilization 5 recently and I've noticed that I can play for hours at a time without even noticing it passing, to the extent that I even forget about meals. The only other game I've experienced that with would be Morrowind. Where I could explore for hours at a time, often not even bothering with the quests.

So I'm curious. With what games, if any, have you experienced this?