Here's what Drupal has for keyword extraction:

1. http://drupal.org/project/int_meta
2. http://drupal.org/project/nodewords

Here's a comparison of these 2 modules: http://drupal.org/node/532278. No. 1, Integrated Metatags, is still in beta but about 707 projects are using it anyway.

Here's 2 lists/overviews of Drupal's most popular modules for SEO:

1. http://kristen.org/content/drupal-seo-modules. (Feb. 2009)
2. http://www.ashowalter.com/content/my-favorite-drupal-6-seo-modules (Jun. 2008)

A presentation given at Drupalcamp, Colorado, on SEO: http://dogstar.org/drupal/content/drupal-6-seo-drupalcamp-colorado (Jun. 2009)

Lastly, a tutorial on SEO optimization: http://tips.webdesign10.com/basic-drupal-seo-on-site-optimization (Sept. 2006)

Lastly, a couple other new SEO modules:

1. http://drupal.org/project/seo_checker (Mar. 2009)
2. http://drupal.org/project/bts (Apr. 2009)