Akismet – spam protection for your blog.
SuperCache – generates static html files from your dynamic WordPress blog.  This will speed you site up.
All In One S.E.O. – a free, easy to configure plugin for optimizing your site for search engines.
DBManager – a must-have plugin for making regular backups of your database.
Fancybox for WordPress – a free, versatile plugin for displaying photos in a mac-style lightbox.