I found out about an old project by Batsly Adams called NESSERIAL/FaMi and immediately bought my first Arduino board. Since I don't have a devcart, I decided to fork his code and impliment a controller emulator. It currently supports keyboard and joystick input from the PC to play NES. The computer end is written in Processing and has been tested on OSX Lion and Windows 7, though it should also work on Linux just fine. I already posted about this on 8bc, but I figure there's more hardware hackers around here that would be interested. Hope you enjoy it!
https://github.com/jummama/ArduiNES
https://github.com/jummama/ArduiNES