There's a pretty cool little board called the IPAC that people use to make computer-based Arcade controllers: http://www.ultimarc.com/ipac1.html.
It's designed so that when you push a button, like the "fire" button on an arcade, it transmits a keystroke like "F" to the host computer. From the host's perspective, someone just pushed the "F" key on a USB (or PS2 -- it works both ways) keyboard.
Mikal