I found this neat site that shows you how to make a nes controller into usb.
http://www.joystiq.com/entry/7817137582525561/ - How to make it
http://www.sealiecomputing.com/retrozone/retrokit.html - Where to buy the kit.
If you like me and not good with reading wire diagrams and connecting up wires you can get a controller thats all ready made for about 10 bucks more. Nes isnt the only controller thats usb supported theres SNES, Genesis, Atari, and Intelevision.