well, I like RegScrubXP. I believe it's for W2K and XP only.
There is also RegClean, it was made by Microsoft, but they discontinued it and stopped supporting/updating it. You can still download it though and it DOES work.
I use them together and some of them find problems that the others don't.
Here is the place that I get all of my free stuff.
www.majorgeeks.com