When I heard about the new Arduino Uno and how it could act as a USB device when connected to a computer, I was really psyched because I've been wanting to make a foot pedal for switching between applications using exposé on my Mac. However, I was a little let down to learn that it's not totally easy to do it (yet!). Anyway, with a renewed desire to make this project once-and-for-all, I realized I could hack apart a USB keyboard and use the parts to do what I want:
I know it's not an Arduino project, but I thought some of the people here would find it helpful or interesting. Hopefully soon a good tutorial will come out for using the Arduino as a USB HID. When that happens, I'm looking forward to making a more advanced programmable foot pedal.