I'm in the middle of a controller build at the moment (lasercut parts just arrived and test fit perfectly!) I highly recommend you look into the arcade and fighting game community sites since they have a wealth of experience on making custom game hardware. Some bits and pieces you might find useful:
Xbox 360 compatibility is a major pain unless you're padhacking (adapting official controller pcbs to custom hardware) because there is a security chip which authenticates official controllers to the system to prevent clone controllers being manufactured. There are a small handful of companies that have worked around it but the upshot is it's not cheap. The most highly recommended encoder is the community developed PS360+ from Akishop
. Otherwise you're pretty much limited to disassembling commercial controllers such as the Joytron Paewang Revolution
or the Qanba Q4RAF
. My current build adapts the Q4RAF. Alternatively, Toodles has created a number of PCBs that "piggyback" an official 360 controller to enable compatibility. This one is based around a PIC microcontroller, designed to be cheap and easy to assemble and works wide variety of consoles including the old sega ones: Universal PCB
This is an interview with Mark Begum a.k.a. Broly who has very limited use of his hands. He plays competitively using a standard controller, operating it with his mouth.
[YouTube]http://www.youtube.com/watch?v=83nSodg-HTU[/YouTube]Headtracking for the 360Xtendplay ergonomic adapter for game controllers
is basically a giant sleeve that makes controllers more comfortable to holdN-Control avenger
adds a bunch of levers that allow you to trigger buttons/triggers by nudging against a bar. Also pre-applies pressure so the buttons act like they're hair trigger.