I know there are quite a few plug-ins that deal with meta tag's (keyword and description) like x-fish, etc - but I would like to suggest some other plug-ins;
1. Blog meta-tags and description = tied to the site's profile - when there isn't a single post (or you're on the index.php page) it should use a Home page meta-tag/description that is editable on the profile page. This is something I'd really like to see.
2. SEO Submission - enable you to submit your site to search engines, etc.
3. SEO user-agent banning - a text-file listing bad user-agents/ips, etc (a perfect ban-list) to dump into a .htaccess