Any webmaster worth their title already knows about the W3C Markup Validation service. They may even occassionally use the CSS validation also (though the results from that can be depressing).
The new tool on the scene is Unicorn - a tool that incorporates all the relevant W3C test tools into one report. It was announced in July 2010
Many of the techniques used to improve accessibility also improve search results and ranking.
Through the use of valid and semantic markup, search engines can better find the important and relevant parts of your content, and your search result placement will improve.