if you mean something like a registry cleaner that actually analyses 64 bit keys, and not just the ones visible to 32 bit applications, I know a few:
- there are the utilities included in windows 7 manager, it does a very deep (or just slow? ) scan of the registry and haven't had any problem with it
- CleanMyPc Registry Cleaner x64 edition - fast and safe
- Bytessence registry cleaner : freeware, there is a 64bit version, appears to work well, but since it is in beta version, I'd recommend to be careful : http://www.bytessence.com/brc.html
For sure there are others, and really a lot that work, but only for keys visible to 32 bit applications.
if you meant something more specific, don't hesitate to add some details