Dive into the possibilities of Microformats from this article at Smashing Magazine. The article points out a few simple additions for your website, that will easily add context to the contents of your site to humans and machines.
An example: If you are selling something, specific crawlers of trade sites will know it as they find the appropriate microformat code on your site. Another example: Make your resume available in hResume format embedded in your regular HTML code and it can be indexed by recruiting companies.
Thanks to Marjolein for the link.