404 Redirected – coupled with SEO Ultimate, allows you to catch 404 errors (ie – outdated bookmarks) and link them to the correct content.

Akismet – comes bundled with wordpress and for good reason.  Best anti-spam tool I have found for wordpress.

Better WordPress Minify  – awesome at reducing the number of HTTP requests your site generates.  combines all JS into 1 <javascript> tag, and all css into a single tag.

Contact Form 7 – simple contact form to email processing plugin

 SEO Ultimate – This one is helpful, not just for SEO but for general site management.  Has a 404 tracker as well as allows you to defined per page custom titles, meta descriptions, and keywords.