Since the best registry cleaners have a backup option, there's no problems if you delete a critical setting in the registry, restore the backup and it's done. I'll endorse PowerTools since I've been using software developed by Jouni Vuorio, the head of Macecraft since 2000, when I got my computer. No problems so far in all those 6 years, not even with MS Office 2000 installed.
Please don't use RegCleaner of Microsoft's RegClean. They're both old, and they could give serious problems with newer computers and software. I tried the last version of RegCleaner months ago, and the program showed some entries in the registry which were dangerous to delete according to my own experience.
Don't use RegSeeker as well. It's very very aggressive and it shows a great deal of entries that could give some problems if deleted. Not to mention the algorithms used to detect useless entries need some rewriting, as it can't detect real paths properly.
EasyCleaner seems like a good option for those using freeware, now if the GUI wasn't so awful... I also use the registry cleaner included in TuneUp Utilities, which complements PowerTools perfectly, and it's also very good, especially for the CLSID entries.
And finally, Mr. Langa could update its comparison to include some other recent registry cleaners like TweakNow's offer (it's not that good, but anyway) and the AMUST Registry Cleaner, which I really don't know if it works or not, since it's only for Win2K and up, but seems like a decent choice.