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.