I've used System Mechanic for a couple of years and I think it works great. The new version 4 is a huge upgrade over the last version. You can get detailed info at http://www.iolo.com
A quick rundown of features:
Tracks down junk files. tmp, bak, chk, dmp files and the like.
Erases all online tracks. I guess this works OK...never really checked up on it.
Finds duplicate files. You have to be careful with this one, but it does work.
Has a built in popup blocker. Haven't used it, but it's probably good.
Has a registry problem fix. I'm hesitant to use programs that fool with the registry like that, but.....
Purge invalid uninstallers. Handy for those programs that leave their tracks in the add/remove programs section after their uninstalled.
Fix broken shortcuts.
Recover delted data. This has a ton of options. I need to play with it sometime to see if it works.
PC maintenance wizard. Can fix some simple problems if you don't know how to do it yourself. Many of the things listed above in one place.
Tweak Windows Settings. This is alot like using the TweakUI utility from Microsoft. Alot of tools and settings that you can't get to very easly through Windows.
Startup Manager. Kind of like msconfig.
Spyware removal. Like AdAware or Spybot. Not sure how it stacks up against those.
Drive scrubber. Wipes drives or disks clean.
Granted, these are the features in the Pro version...not sure about other versions. But it's a handy program for alot of tasks.