I make sure I use these on ALL my WordPress blogs. I use many more, but this is a list of plugins that I think anyone who is serious about maximising the potential of their blog should be using.
All in One SEO Pack
Google Analytics for WordPress
Google XML Sitemaps
TweetMyBlog!
wp-cache
Add to Any: Share/Bookmark/Email Button
YARPP
If you have others that you believe are essnetial and should be mentioned then leave a comment and I shall consider updating this list.
I like Cooliris’ wordpress plugin – http://developer.cooliris.com/?p=wordpress
not so much SEO as a wow factor, especially for sites hoping to add a “Web 2.0” meme to their posts