I just thought I'd give a little insight to those who are looking for retro game pads that plug directly into the USB port. I got a couple NES and SNES ones from Retrozone (http://www.retrousb.com/) and they are phenominal. I absolutely love playing old games with the controller and these are USB, so they are plug'n'play, easy to configure, and you can have more than one at a time.
They have controllers for NES, SNES, Virtual Boy, Atari, Genesis, Jaguar, Intellivision, Colecovision, Vectrex, Turbo Graphix 16, 3D0, Bally Astrocade, Atari 5200, Sega Saturn and Neo Geo.
Only thing missing is the N64, but I've looked long and hard into it and can't seem to come up with anything except those PSX/N64 adapters that don't quite work anyways.
Anyways, at about 30 bucks they can be a bit pricey, but definitely worth it if you as big on nostalgic feel as I am.
That site again is http://www.retrousb.com/