PHP W3C Valdiator is a PHP script that can be installed on a webserver to validate the HTML/XHTML and CSS of an entire site. It recourses through the sites directories checking for web pages, then checks them with the W3C site for validation errors. The script opens a direct link to the W3C sites so is always up to date with the latest checks

It also can support sessions, which the standard validator struggles with, allowing sites with logins or session dependant scripts or dynamic CSS to be validated properly. Finally it also has an option to test multiple remote scripts And of course it's free :) You can download PHPUSB from sourceforge.net Current Version Download 1.1 phpw3cvalidator1_1.zip