This video is just flat out amazing. Mike Phillips is a Mac gamer and writer, who just happens to have been born with spinal muscular atrophy. He's apparently only able to move his thumb, and yet that doesn't stop him from playing Unreal Tournament and World of Warcraft. This is possible thanks to a proximity switch connected the Swifty USB switch interface from Origin Instrument and the SwitchXS software from AssistiveWare. The latter looks basically like a kind of on-screen keyboard, that Mike scrolls through to select arrows or letters, etc. Seeing Mike frag people left and right is enough to make me keep away from him online.
AssistiveWare has a variety of products to help disabled Mac users, as well as other videos demonstrating some of them.