WAVE–the Web Accessibility Evaluation Tool–is a free and extremely user-friendly online service to allowing you check, validate, and optimize your web site design and structure.
Instead of providing a complicated-to-understand and hard-to-read textual report on what discovered issues are, this tool displays the page you are checking in and puts small icons with observed issues and notes near the questionable component of the design. With that you will see the problems in contest and formatting very clear, and you will be able some time to get everything fixed. You can instantly find and fix pointed issues in your code.
If you are still in process of building your page, you can also upload individual files, or even paste code into the form, posted on the website, and let it be checked on the fly.
WAVE is a very useful service, especially for newbies, it is free, and it does not require registration.