I own Soul Reaver on every format it was released on. It's not even one of my favourite games. I do like it, but I have no idea what possessed me to buy it so many times.
I originally owned Ocarina of Time on the Zelda: Collector's Edition on the Gamecube, then bought the 3DS version a few months ago.
I own Hitman 2 on the original Xbox and have owned Sonic Adventure 2 on the Dreamcast, Gamecube and PC. I've owned the original on Gamecube and Dreamcast, too. I also rebought Psi-Ops for the Xbox when my PS2 copy broke.
I bought The Witcher 2 in a Steam sale after losing my retail copy.
I have several games on both PS3/360 and PC. I barely got to play the PS3 version because the PS3 and 360 aren't mine, so when I got my new PC I rebought them. Games include: Sonic Generations (I also own the 3DS vrsion, but it's a different game, really), Prototype, Dragon Age, Dungeon Siege III, Skyrim and GTA IV (I bought the entire series in a sale, despite owning all of them on consoles and even having the PC version of 2 already)
My game buying habits are terrible...