Akismet - Is a spam filter that checks your comments against the Akismet web service to see if they are spam or not, also checks the trackbacks for spam.
Ultimate Tag Warrior - Best Tag system for WordPress. You can use categories and tag system on one website which is very cool.
Exec-PHP - Execute PHP code in posts or static pages. When this plugin is on you can add php code to static pages or even posts.
Google Sitemaps - This generator will create a Google compliant sitemap of your WordPress blog.
No Ping Wait - Speeds up posting by moving generic pings to execute-pings.php.
PXS Mail Form - Creates a mail form with multipart verification, various messages and an auto redirect on successful send. It is much more easy to contact with website author.
Super Archive - Implements a dynamic archive, the best archives system i know.
SRG Clean Archives - Super clean archive. Very simple and useful.
WP lightbox 2 - Overlay images on the current page for WordPress. You can turn it on and all images are now working with it.
fQuick - Sidenotes, short notes for WordPress. You can place it for example in sidebar.
flickrRSS - Integrate the photos from a flickr rss feed into your site. Show flickr photos on your website.
Feedburner Feed Replacement - Forwards all feed traffic to Feedburner while creating a randomized feed for Feedburner to pull from.
Gravatars - Adds a nice looking graphic next to the persons comments.
Get Recent Comments - Display the most recent comments or trackbacks with your own formatting in the sidebar or where you want.
WP Paginate - Create pagination for pages. It is the best plugin for that makes navigation easy with many static pages.
wp-notable - Add social bookmark links such as digg or del.icio.us to each blog entry
Adsense-Deluxe - Support for AdSense ads and allows you to add ads to posts.
WP-ContactForm - A way for people to contact you without actually emailing you, saves time, and great way to avoid spam.
PodPress - Got a podcast? PodPress makes it really easy to post podcasts, also includes an inline media player.
Related Posts - Link to related posts on your blog, which is based on the content of the post.