Free Web Tools - SearchBliss Web Tools for the Webmaster - I like the links on the SEO tools page.

Google Webmaster Guidelines - Following these guidelines will help Google find, index, and rank your site. These are also good guidelines to make the site accessible.

Google Search Engine Optimization (SEO) Guidelines - This article is directed at a website owner looking to hire a Search Engine Optimizer.

W3C HTML/xHTML Markup Validation Service - This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML. It's best to have web pages validate for xHTML so they are compliant with the modern browsers, and Google may give the edge to a website without a lot of errors in the code. I believe Javascript errors aren't as bad as improperly nested tags for instance.