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)
As mentioned above, the Nodewords module is your best (IMHO) option for Drupal 6; just make sure to use 6.x-1.13 or newer rather than the older (horribly, horribly broken) versions.
For Drupal 7 just go with the Meta tags module; we're actively working on getting it stable and are very close.
Disclaimer: I am the maintainer of the Nodewords module.
I have used Yahoo auto tag module in the past. It worked well for me. Not sure why only 10 people have used it in the past.