The following resources contain additional information on SEO. Please use them to get more in-depth knowledge on this topic.
Google Webmaster Guidelines - A list of guidelines which will help you to optimize your site for googlebot.
SEO Article from Wikipedia - A nice article on SEO from Wikipedia. It has other useful links as well.
SEO Professionals - This is is non-profit organization of search engine marketers founded in April, 2001.
Wiki Page for SEO related Concepts - Find many other concepts related to SEO.
SEO Resource Directory - A list of various useful SEO resources.
Search Engine Spider Simulator - Check how your site will be crawled by a search engine web crawler.
Keyword Density Checker - Check your keyowrds density on your web page using this outstanding tool.
Keyword Suggestion Tool - A tool from google which suggest you various keywords when you type a word.
HTML/XHTML Validator - Check if your site is compliance to W3C standard or not.