Download Macromedia Contribute. If your web site is enabled for this tool, you may use this tool to take control of your web site content without the need for a full Content Management System and all the cost overhead that entails!
Cynthia Says is an accessibility assessment tool that you can use to test single web pages according to US Section 508 requirements or WCAG Priority 1, 2 or 3 level.
WebXAct. They say:
WebXACT is a free online service that lets you test single pages of web content for quality, accessibility, and privacy issues.
W3C Markup Validation Service. They say:
This is the W3C Markup Validation Service, a free service that checks Web documents in formats like HTML and XHTML for conformance to W3C Recommendations and other standards.
W3C CSS Validation Service. They say:
Welcome to the W3C CSS Validation Service; a free service that checks Cascading Style Sheets (CSS) in (X)HTML documents or standalone for conformance to W3C recommendations.