Under Windows and Linux, the mouse scroll wheel seems to be linear. Under Mac OS the scroll wheel some sort of acceleration which means it either goes too slow or too fast.
USB Overdrive does this as well, and it is shareware. It can also remove the acceleration added to mouse movements.
Apparently SteerMouse lets you change scroll wheel settings but it is not free software.
By paying for it you ensure that there's somebody out there who cares about the mouse scrolling experience on Macs.
(disclaimer: I'm the developer)