Plugin allowing you to turn on gravatars for your site. For those which do not know about gravatars - this is icons which are linked to your identity account at the site gravatar.com, thus in your blog you and your users and guests could have their own gravatars
http://skippy.net/gravatars/

five-wordpress-cms-enabling-plugins - really good article on the plugins allowing you to consider wordpress as the real CMS for your site
http://blazenewmedia.com/articles/five-wordpress-cms-enabling-plugins/

Trackbacks
Explanation what is it and how to turn it on for your blog. Very usefull.
http://codex.wordpress.org/Template_Tags/trackback_rdf

PingBack

Pingback lets you notify the author of an article if you link to his article (article on a blog, of course). If the links you include in an article you write on a blog lead to a blog which is pingback-enabled, then the author of that blog gets a notification in the form of a pingback that you linked to his article.

Trackback

Trackback helps you to notify another author that you wrote something related to what he had written on his blog, even if you don’t have an explicit link to his article. This improves the chances of the other author sitting up and noticing that you gave him credit for something, or that you improved upon something he wrote, or something similar. With pingback and trackback, blogs are interconnected. Think of them as the equivalents of acknowledgements and references at the end of an academic paper, or a chapter in a textbook.

How to block or allow Googlebot?
http://www.google.com/support/webmasters/bin/answer.py?answer=40364&ctx=sibling

Block or remove pages using meta tags
http://www.google.com/support/webmasters/bin/answer.py?answer=61050&ctx=sibling

Resource devoted to the information on robots.txt file
www.robotstxt.org

How do I add my site to Google’s search results?
http://www.google.com/support/webmasters/bin/answer.py?answer=34397&ctx=sibling

What’s an SEO? Does Google recommend working with companies that offer to make my site Google-friendly?
http://www.google.com/support/webmasters/bin/answer.py?answer=35291&ctx=sibling

Google searches more sites more quickly, delivering the most relevant results.
http://www.google.com/technology/index.html

Search_Engine_Optimization_for_Wordpress - “native” wordpress team built article. Great. Outstanding. With lots of references to the very useful plugins. Must read!
http://codex.wordpress.org/Search_Engine_Optimization_for_Wordpress

Have not used this plugin but take a look at the list of its abilities:
1. Edit-in-place
2. Ability to backup write panels and import them
3. Date variables will be sortable via query_posts
4. A powerful query_posts function that will allow you to use FreshPost variables
5. Write panel organization (order of fields/panel)
6. Grouping fields together in the write page
7. A new admin theme for FreshPost
http://freshout.us/goodies/fresh-post-for-wordpress-wordpress-cms/

date-exclusion-wordpress-plugin - may be usefull ig you would like to exclude dates from the wordpress pages. it could be valuable for search engines optimization!
http://www.dailyblogtips.com/date-exclusion-wordpress-plugin/

Mod of wordpress having only functionality for not expirienced editors. Still - there are another plugins allowing you to controll all the abilities of the certain users. Still could be usefull to someone.
http://pixopoint.com/simplecms/

WordPress Trackback Tutorial
http://www.optiniche.com/blog/117/wordpress-trackback-tutorial/
http://faq.wordpress.com/2006/05/11/how-do-i-send-a-trackback/

Sending Pings and Trackbacks
http://www.tamba2.org.uk/wordpress/ping/

Using_Permalinks
http://codex.wordpress.org/Using_Permalinks

permalinks-migration-plugin - could be usefull if you decided to migrate to another permanent links structure, but would like to save the old-style pages accessible for the external resources. Potentionally plugin I would install for zmicer.com to migrate into permanent links style of page links constructing.
http://www.deanlee.cn/wordpress/permalinks-migration-plugin/

tutorial-automatic-sitemap-in-wordpress - three cases is considered. Must to read.
http://www.jimwestergren.com/tutorial-automatic-sitemap-in-wordpress/

seo-for-wordpress-blogs
http://www.jimwestergren.com/seo-for-wordpress-blogs/

SEO optimization
http://www.tomrafteryit.net/search-engine-optimisation-step-1-identification-of-aims/
http://www.tomrafteryit.net/search-engine-optimisation-step-2-benchmarking/
http://www.tomrafteryit.net/search-engine-optimisation-step-3-keyword-and-keyphrase-research/

using-headspace-wordpress-plugin-for-seo -SEO in action.
http://www.webhelpermagazine.com/2007/10/using-headspace-wordpress-plugin-for-seo/

WordPress SEO - The beginner’s guide
http://www.joostdevalk.nl/wordpress-seo/