i'm using a kensington something or other. it's backlit blue... extremely cool. 5 buttons, and the kensington mouseworks software (not required, but i love it) makes it so a 'chord' is a button (ie pushing two buttons at once). also you can make the buttons do key macros and each button can do something on a per-application basis:
example:
i right-left click to delete a file in the finder. right-left click copies text if it is highlighted, and the middle button pastes.
the back button on the mouse moves back in Safari (which is command-[) and also moves back in NoteTaker 3000 (which is command-left). it takes a little bit to set that part up, but once it's done it saves so much time.
of course you can just plug it in and it'll work as a regular ol' 5 button mouse anyway if you want it too. i just thought i'd reiterate that all that extra crap isn't required if you don't want it ^_^.