404 Redirected – coupled with SEO Ultimate, allows you to catch 404 errors (ie – outdated bookmarks) and link them to the correct content.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