Keep in mind that these tools are not designed to 'fix' your computer, as oil changes and tire rotations are not designed to fix your car... However, these tools might help to keep your computer clean and tuned up. As virtually any other AI designed by human beings these tools are capable of making wrong decision and remove things that should have been left alone - use with caution and care!
My favorite has always been CCleaner - simple and effective all-in-one tool with free version available for PC and Mac. Most of IT professionals I know prefer to use this tool on their own computers.
A bit more featured, yet solid and user friendly product is System Mechanic. Also comes as free version which can be upgraded to version with advanced features (if necessary). If CCleaner preferred by pros System Mechanic would probably be a choice for end users.
More to come...