There's a good test comparing the effectiveness of 10 reg cleaners
here.
He ends up suggesting that
EasyCleaner is the best free tool.
Notwe that his test results were published 9 months ago, so things may have changed since then, I don't know.
I use (I paid for license) Rose City Software's Registry 1st Aid myself, and am happy with its performance, although it doesn't place well in the above comparison test. Once I tried the popular jv16 reg cleaner & hosed my system with it because I was too aggressive in using it.