I wrote before about how to use your iphone as a game controller to use with a snes / playstation emulator on your mac

the app that I used then has been replaced, with a new version that only works with ipad games, unfortunately :(
they do have a ‘legacy’ version that does still work, but, there are a few apps that have popped up in their place:

wifipad is very cool – its a free app for your iphone. It has a server component that runs on your desktop machine that translates from the app. the server component runs on mac, linux and windows!