CCleaner is also essential for cleaning up poorly uninstalled apps leaving registry keys lingering around. It's also great for when you need to fully reset and reinstall a misbehaving app, such as Symantec Endpoint Protection, which fails to clean up several critical registry keys that it checks during installation, and then refuses to re-install if they exist. Sure, you could hunt them down and write a script for Symantec, but CCleaner does it for all apps. I've fixed so many terrible apps and installs using CCleaner, it's an absolute necessity.

That and RevoUinstaller. You run RevoUinstaller then CCleaner, and maybe a reboot in between, and you've really removed an app. (The paid version of Revo is even cooler, as it will let you log the files generated during an install, and then on uninstall, it'll make sure to remove all the files and directories you had previously logged.)