The free ones I've come across do not offer a complete test of your complete site. Typically they give you a quick check-up. Such free web tuners cannot replace the live testing of people who are willing to provide feedback. People have different tolerance and comfort levels. People have different levels of curiosity.
People have different skill levels.
While you can assume that most people know how to point and click, do not assume that they know web surfing tricks such as point, right-click, and open target in new window. One visitor got stuck when a link was opened automatically in a new window. Identify and map out people who would be willing to test your site. As you fix and improve your site, you will need to rely on people who will continue to support your efforts. Count on your immediate family first.