I use RegSeeker http://www.hoverdesk.net/freeware.htm
It is free and has a bunch of tools in it
From the makers of HoverDesk http://www.hoverdesk.net/

Yep, RegSeeker is good one.

I can recommend RegHeal: http://www.fixregistry.com/index.htm

Not free, but good value IME. Been using it on few systems now with no problems.

I use EasyCleaner by Toni Arts. It seems to work well, but frankly, I really wouldn't know. It's free, nice GUI and has a few more tools with it. I downloaded it from http://www.majorgeeks.com/download414.html

I have used system mechanic in the past. But be warned, with any program that messes with the registry...
YOU CAN EASILY CRASH YOUR SYSTEM! System Mechanic has done it to me twice, and other reg cleaners have caused me grief as well.

I use CleanMyPC Registry Cleaner, also a good one

I use Advanced System Optimizer, I like it so far, have had it for about 2 years.

PCmag.com recommeded VCOM Fixit Utilities 5.0. I got it for under $20 at nothingbutsoftware.com. After a month, I haven't had any problem. I have cleaned my registry about 3 times because I'm always installing and removing programs. It's got other programs but I have Norton's System Works and didn't want to use the disk cleaner so didn't install it.

PCmag.com recommended VCOM FixIt Utilities 5.0. I got it about a month ago from nothingbutsoftware.com for under $20. It's usually $50. I've cleaned my registry 3 times since then without any problem. I really trust it and would be willing to pay more. There's also a utility for compacting the registry. I didn't install the disk cleaner. I have Norton Systemworks.

If you already have SystemWorks, Clean Sweap is a good pogram.  If you don't, don't buy it just for that, use Reg Seeker.

I like TuneUp Utilities 2004. Fast, safe and effective. I disabled the winstyles service that it installs (and wasn't part of TU2003). I don't have proof, but I suspect the TuneUp WinStyler is a hacked uxtheme file just like StyleXP.

Jeff