bbPress is forum software with a twist from the creators of WordPress.
Q&A allows any WordPress site to have a fully featured questions and answers section.
Wiki allows you to create a wiki as easily as adding a post.
WP Help allows administrators to create detailed, hierarchical documentation for the site’s authors and editors, viewable in the WordPress admin.
AddToAny: Share/Bookmark/Email Button helps people share, bookmark, and email your posts and pages using any service, such as Facebook, Twitter, Google Buzz, Digg, Delicious, and well over 100 more social bookmarking and sharing sites.
MarketPress brings you a solid, simple yet extensible foundation for a WordPress-Powered store.
WordPress PopUp allows you to display a fancy popup (powered as a popover!) to visitors sitewide or per blog.