Other than the obvious answer of Pokemon:
Kingdom Hearts (1, CoM, 2): Oh. My. God. No normal human being can ever get enough of Sora, Goofy and Donald, and of course a little Riku on the side. These series of games are easily my favorite, and I would spend hours doing whatever little post game content there was. I've replayed the first one and CoM a couple of times and would definitely do the same for KH2 if I still had my PS2. I imagine myself wasting countless more hours on 358/2 Days and Birth By Sleep when they're released.
Tales of: Tales of whatever, it doesn't matter. As long as it has those two magical words in the beginning, I have most likely clocked at least 50 hours in that game. I still remember the time at my sisters house where I literally stayed up for 48 hours straight just to beat Tales of Symphonia. I had quite a few hours clocked on Tales of the Abyss as well, and I haven't even beaten the game yet! (Tales of Vesperia was surprisingly quick, though)
Call of Duty: I'd like to meet the FPS player who doesn't spend hours playing this beauty. THEY COME FOR OUR BLOOD, BUT DROWN IN THEIR OWN!