CCleaner

CCleaner

From the CCleaner main page:

CCleaner is a freeware system optimization and privacy tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. But the best part is that it's fast (normally taking less that a second to run) and contains NO Spyware or Adware!

I've been using CCleaner for about a year, and although it is a powerful tool, the interface is somewhat confusing. Does a very thorough job and has a lot of useful features. I see they've gotten away from the original name of the product, "Crap Cleaner".