Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog's "Comments" admin screen.
http://wordpress.org/extend/plugins/akismet/

The Google Analytics for WordPress plugin automatically tracks and segments all outbound links from within posts, comment author links, links within comments, blogroll links and downloads. It also allows you to track AdSense clicks, add extra search engines, track image search queries and it will even work together with Urchin.
http://wordpress.org/extend/plugins/google-analytics-for-wordpress/

Yet Another Related Posts Plugin (YARPP) gives you a list of posts and/or pages related to the current entry, introducing the reader to other relevant content on your site.
http://wordpress.org/extend/plugins/yet-another-related-posts-plugin/

Automatically add links to your favorite social bookmarking sites on your posts, pages and in your RSS feed. You can choose from 99 different social bookmarking sites!
http://wordpress.org/extend/plugins/sociable/

This plugin will create a Google sitemaps compliant XML-Sitemap of your WordPress blog.
http://wordpress.org/extend/plugins/google-sitemap-generator/

This Flickr plugin for WordPress will allow you to pull in your Flickr photosets and display them as albums on your WordPress site.
http://wordpress.org/extend/plugins/tantan-flickr/

Wordpress Automatic Upgrade allows a user to automatically upgrade the wordpress installation to the latest one provided by wordpress.org using the 5 steps provided in the wordpress upgrade instructions.
http://wordpress.org/extend/plugins/wordpress-automatic-upgrade/

SEO Friendly Images is a Wordpress optimization plugin which automatically updates all images with proper ALT and TITLE attributes. If your images do not have ALT and TITLE already set, SEO Friendly Images will add them according the options you set. Additionally this makes the post W3C/xHTML valid as well.
http://wordpress.org/extend/plugins/seo-image/

WassUp is a new Wordpress plugin to track your visitors in real time. It has a very readable and fancy admin console to keep tracks of your blog's users visits.

Automatically optimizes your titles for search engines Generates META tags automatically Avoids the typical duplicate content found on Wordpress blogs
http://wordpress.org/extend/plugins/all-in-one-seo-pack

Focuses on just the most popular metrics a blogger wants to track and provide them in a clear and concise interface.
http://wordpress.org/extend/plugins/stats/

Tired of copying and pasting the embed HTML from sites like YouTube? Then this plugin is for you.
http://wordpress.org/extend/plugins/vipers-video-quicktags/

Plugin Manager lets you to view, download and install plugins from wordpress.org from an AJAX'ed interface, instead of manually downloading, extracting and uploading each plugin.
http://wordpress.org/extend/plugins/plugin-manager/

WP-Polls is extremely customizable via templates and css styles and there are tons of options for you to choose to ensure that WP-Polls runs the way you wanted. It now supports multiple selection of answers.
http://wordpress.org/extend/plugins/wp-polls/

Adds a more advanced paging navigation your WordPress blog.
http://wordpress.org/extend/plugins/wp-pagenavi/