From Hamsterworks Wiki!
This Arduino project made a jumbo switch act as a space bar, allowing my disabled son to play simple games like "KneeBouncers" or "Sights and Sounds Press". The project uses the virtual keyboard project from http://www.practicalarduino.com/projects/virtual-usb-keyboard with only a few lines of custom code.
The end result works well in desktop PCs, but is pretty hit-and-miss in laptops and netbooks - the USB device does not detect correctly. I can't understand why, but I haven't looked into it too closely either.... maybe the voltage divider is a little out of spec.
The other shortcoming is that you mustn't press the switch while the soft USB stack is initialising, making it a little tricky when somebody is eager to play with it!