You have Back, Forward, Stop, Refresh, Search, Favourites, Web/Home, Mail, Mute, Volume +/-, Play/Pause, Stop, Prev Track, Next Track, Media, My Computer, Calculator and Sleep.
Obviously you can program which browser and mail software the buttons use. The Media button brings up a list of all your media software (surprising that? 🙂). The media controls (play, stop etc) work in the currently running media app.
The My Computer and Calulator buttons are programmable, buts that all. But what else would you program in that isn't already there?