There are quite a few good ones out there, but in my opinion at the top of the pile is NBG CleanRegistry by Nobody Group. Finds a lot more redundant entries than most cleaners, also has never taken away any important entries by mistake. I always use this, top quality stuff.
I think you will struggle to find a half decent free cleaner, I may stand corrected but my experiences with freebies in the past have led to format c:\ !!!!! You have to pay for quality in my opinion.
look under my username in the search and oyu will find a question from me asking about a link to microsoft reclean that i lost and they no longer support.
Its the best i have used ever and give u the change to back up incease u remove something u shouldnt have