Decelerator is a small Mac OS X application that allows you to customize the acceleration setting of your mouse or trackpad.
Gamers often disable mouse acceleration entirely to gain a competitive edge in "twitch" reflexes. With mouse acceleration off, a distance on-screen corresponds with an exact distance your hand must move, and is unaffected by the speed with which it moves there.
Windows users often find Mac OS X's relatively high mouse acceleration disorienting and frustrating. Lowering the mouse acceleration can go a long way to easing the transition to a new operating system.