I’ve tested a lot of them and can drop some words:
CCleaner: very superficial, but safe. No problems.
Amust Registry Cleaner: Hmmm, I don’t feel comfortable using it. Uninstalled quickly…
Eusing Free Registry Cleaner: good and safe.
jv16 Powertools 1.3.0.195 (last freeware version): old, but functional.
ToniArts EasyCleaner: good and safe.
Wise Registry Cleaner: the best I’ve tested. Safe and deeper.

Other registry tools (not cleaners):
Auslogics Registry Defrag
ERUNT: backup must have.
RegASSASSIN: delete access denied keys.
Registrar Registry Manager Lite: registry explorer (replaces regedit.exe with more features).
RegShot: compare registry shots, for instance, before and after installing some software.