A few I put on every blog I build:
WP-DBManager - A must have for EVERY blog. Backs up your DB according to your specifications. Never worry about loosing data to a DB Crash again.
WP Auto Tagger - Auto tags your posts.
Google XML Sitemaps - Builds and updates your sitemap for google.
All in One
SEO Pack - My favorite
SEO Plugin.
Also if you haven't changed your permalink structure it's easy to do and a BIG help for
SEO:
In the dashboard click Settings>Permalinks, change it to a custom structure with:
/%postname%/
This changes the link structure from something like this:
mydomain.me/?p=124
to something like this:
mydomain.me/this-is-my-postname/