Recently, it's been Fallout 3, and I've been replaying through The Witcher as well, although it's hard to call that a black hole because, while lengthy, it doesn't take forever to finish (about as much as the Knights of the Old Republic games, maybe a bit more). In the past, my biggest time sink was undoubtedly Diablo II... more time was wasted playing that online during my teenage years than any other game, period. I gave it up after several years because I found myself economically unsuccessful next to all the hackers around me, and my character builds were all disappointing despite the time I had put into them. It seemed too daunting to start all over again, so I stopped entirely. Can't say I regret it, really.