USB MAME
From Fozzee.net
Back in the day, I wanted a controller to play MAME on my PC. I happened to have a Saitek "left hand" controller laying around that had a 4-way hat, and a number of buttons. I built a console box out of a piece of 1x4 and a small bit of plywood.
The joysticks and buttons are ordered from Happ Controls. Their stuff ranges from super cheap (my stuff) to super expensive, competition optical joysticks. The only regret I have is that there's something up with my underlying controller that dislikes certain chording. Shoulda sprung for an IPac.
If you're looking to do this to other controllers, consider the pinouts for those controllers on this Russian page:

