I ashamed that I had to crawl out of the bowels of the software section to answer this one. You guys should know better.

It's called directpad pro (or ntpad for you xp users) and it uses a very simple parallel port interface. (like two resistors and that's it... even those are optional) There is actually a section devoted to this. On this very site.
I know that the parallel port is virtually a dead avenue at this point, but for now it's still the best bet to connect classic console pads to the pc.
Even if you are good at usb programming, the interface, the usb controller chip and the cables are gonna run about 30 bucks. A usb chip programmer is gonna cost another 20-30 bucks. Then you have to actually know how to program the dang thing.
Parallel port ends sell at radio shack for a whopping 3 bucks.
So if you are willing to spend 60 bucks to conect a pad that has a current retail value of about $1.50 then go for usb, otherwise, use the parallel port connection method.
