Trackballs

Much like Cachecrash I've had a trackball for many years now, and I can never go back. It did take some getting used to at first, but again, not more than a few hours. And personally I think they're vastly more efficient when playing most games, particularly any first-person game, although a bit more awkward with most real-time strategy games.

The two kind of trackballs I've had experience with are the Logitech TrackMan Wheel and the Logitech TrackMan Marble. However, I'm not so fond of the Marble - they're almost flat and the marble gets dirty easily. I vastly prefer my Wheel because it's arched up a lot higher, which is far more comfortable on my hand, and the marble is designed to not get as gunked up so easily.

Jam it back in, in the dark.