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Free/open source would, of course, be preferred. I'm willing to hear of paid options too though.
A specific problem requires a specific solution. If you don't have a specific problem, leave the registry alone.
Whilst software may leave junk behind when uninstalled, it usually does no harm. If a rogue program has done something you don't like (such as changing file descriptions) you can make corrections manually.
As Kaled says, you probably won't notice a huge improvement from using a registry cleaner. However, I would have to say that it does offer performance benefits over the long haul.
Removing junk may improve speed but the difference is likely to be marginal rather than noticeable. A significant improvement is likely only if there is a specific problem, and if such a problem exists, unless it is a known and common fault, a registry cleaner is unlikely to fix it.
The registry is a free-format hierarchical data-store. Its nature is such that generic cleaning is mostly impossible therefore any cleaner must rely on rules. If the rules are wrong, damage will result. Certain generic errors can be fixed such as unterminated strings, but such problems are rare (and are probably detected and fixed automatically by Windows, but I'm not certain).