CCleaner, a.k.a. CrapCleaner for Windows is a tiny application which does just that, cleans crap from your system. That means “fixing issues in the registry” , removing stray temp files and useless installers, clean cache and so on. Upon first run it cleaned 500 mb off my system, a system I thought fairly optimized.
If you’re a Windows user, this’ll speed up your system. Just make sure you don’t accidentally delete all your cookies in the process, unless you actually want them gone.