I got fed up with the jumpy 'cliff-edge acceleration' mouse movement on my Mac. Not wanting to spend any money on USB Overdrive just to fix a basic problem like this, I wrote a little program called MouseFix that makes the mouse much more usable. It's free and open source; I wrote it on a 10.4.1 system, but I would guess that it will probably work fine on virtually any version of OS X, if not all.
[robg adds: I know many people dislike the new mouse acceleration curve that was introduced with (I believe) 10.3. I personally disliked it as well, but thankfully, the Microsoft drivers for my mouse override it. If you're using a stock mouse, though, and are looking for a solution, this program may be it (I haven't tested it, though).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050724132607997