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.