I admit I'm a sucker for specialty peripherals:
-2 Cobalt Flux PS2 DDR pads (60 pounds each--tough to schlep out for my kids)
-PS2/XBOX Karaoke Revolution microphones
-PS2 Singstar microphones
-PS2 Guitar Hero guitars
-Dreamcast Densha de Go (train) controller
-Dreamcast Pop 'N Music controller (the smaller, cheaper one)
-Dreamcast Fishing rod (that I still use)
This doesn't include the classic console peripherals like the Arcadia Supercharger, KidVid tape player, Joyboard, the NES Power Glove, etc (all of which I parted ways with long ago).
However, I have not plunked for Wii Fit or Rock Band. Not sure if I will either. Too much stuff already in the closet