Last I heard there are three.  The first is a 2 button one that looks just like the standard ms wheel mouse.  At the same time that huge silver guy came out.  Just a few months ago the released third one, inbetween the other two in price or function.  4 buttons, plus the wheel.  I've had one for a month now and love it.  Very precise, never needs cleaning.  I don't play a lot of fps games but it seems to do ok for the odd quake2 that I do do.  It took a few days to adjust to it but now I wouldn't trade it for anything.