How can I automatically add taxonomy terms to a page's  keywords in the HTML? When Taxonomy is used for "free tagging", it basically becomes keywords for the content. From an SEO perspective, it would be great to automatically include these taxonomy terms in the  HTML tag for keywords, letting users help drive your SEO, or giving a non-technical site maintainer the ability to easily edit the keywords used on any given node. How can I do this in Drupal (both 6 and 7)
I think you are looking for the Drupal 6 Nodewords module.

The Meta tags module is/was supposed to be the new Drupal 7 version, but it appears to be on hiatus. The Meta tags quick module, however, is an option (currently in dev).

This helpful post has more details.