W3C Validator (HTML)
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available.
http://validator.w3.org/

W3C Validator (CSS)
Om du vill validera din CSS-formatmall inbäddad i ett (X)HTML-dokument, så bör du först kontrollera att den (X)HTML du använder är giltig.
http://jigsaw.w3.org/css-validator/

W3C Validator (Checklink)
This Link Checker looks for issues in links, anchors and referenced objects in a Web page, or recursively on a whole Web site. For best results, it is recommended to first ensure that the documents checked use Valid (X)HTML Markup. The Link Checker is part of the W3C's validators and Quality Web tools.
http://validator.w3.org/checklink

XML Validator
Allows you to check your XML documents on conformance to W3C specifications for XML 1.0. The validator provides document check, including grammar your document points to.
http://www.validome.org/xml/

XML - DTD and Schema Validator
Checks syntax and structure of your XML DTD or Schema according to W3C approved recommendation. The validator provides check of the document you specify.
http://www.validome.org/grammar/

Validator for Atom and RSS feeds
This tool provides validation of your Atom and RSS files based on XML schema validation; as not all requirements of appropriate specifications can be expressed in a XML schema, we provide additional checks to insure the conformance of documents to check to exsting RSS and Atom specifications and standards.
http://www.validome.org/rss-atom/

Google Sitemap(s) Validator
Provides validation of your Google Sitemap and Sitemap for Sitemaps: Google provides this service for helping webmasters to get even huge websites better crawled. The sitemap you create must be well-formed XML and fulfil some more, specific requirements.
http://www.validome.org/google/