Logitech Mouseman+ PS/2, from before the time when mice lost their balls. I've learned how to take care of it and having used a lint roller on my mousepad, I've only needed to clean it twice in over two years (in fact, looking at the rollers now, they're not the least bit dirty - and I haven't cleaned it in a year). That's a really good thing, considering I really can't imagine losing this thing. It's far and away the best damn mouse I've ever used.