World of Warcraft is the obvious answer.
I'm sitting here at work right now, and all i can think about is leveling my mage, and having brain freeze proc and firing off a mana-free instant fireball. OH cool down on my elemental is up, come on out big guy! Free long distance frost nova incoming! BLIZZARD!

I can't wait to get home.
Every couple of months i'll get the urge to play "Advance Wars" on the DS.. It always plays out this way.. The new AW game comes out, i play it, am a bit disappointed that it's not as fun as i remember the previous entry being, and i put it down for a couple of months.. Then on a whim i pick it back up and get re-addicted to the series all over again and play it right thru to completion, then i usually play thru the hard modes and get all of the unlockables..
Also i always have urges to BUY new games, even though nowadays i don't have the actual time to PLAY them.. It's a real problem i have.. Right now i want to buy "Left 4 Dead 2" and "Prototype", but i know darn well i'll probably play them both for a week and then just drop them, but gosh i really want to buy them..