I'd forgotten about this page, although it's been kicking around for ages. There's some source code for the CY7C63001A-WC processor. There are two varients of the code, one for PSX and one for N64.
For those who want to attempt to build their own converters, this would be a good place to start. You'd need to rewrite the input side to handle a joypad other than a PSX or N64, but that's up to you to do! ;)
The link: http://www.zen35383.zen.co.uk/stnield/usbpads.htm (http://www.zen35383.zen.co.uk/stnield/usbpads.htm)
It might be worth mirroring this at some point in case it goes away!
Good idea... the site they link to for buying pre-programmed chips has been shut down:
"This site is currently under investigation
and has been removed from this server
Any emails sent to this address will be investigated"
I noticed that, but decided that code was of more value than the preprogrammed chips. After all, it's far easier to work with the source code than try to reverse engineer a preprogrammed chip. If the code protection flag is set on the chips, it can be very difficult to get the object code back out of them!
My point was, someone mirror the site ASAP, in case it too is removed from the server for investigation...!