I agree with everyone that mentioned Pokemon. I have a compulsion to buy all the Pokemon games, even though they're all basically the same; I wish I was sensible and could limit myself to buying one game from each generation, but no. Somehow I end up with every game of each gen, despite the fact that they're very much the same, save for some differences in which Pokemon are exclusive to each. Still, at least they're good games which can be pretty addictive, even if they are repetitive in places.
Also, the Sims. I hate EA for the most part. I don't even play the games. (I used to years ago when I was younger, but I'm out of that phase now.) Still, I bought the Sims 3, just because I felt that it was necessary to own it. I played it for about an hour and then forgot all about it. This is why I will never be good with money.