Debug Bar adds a debug menu to the admin bar that shows query, cache, and other helpful debugging information. Requires WordPress 3.1.
Graceless Degradation gives visitors still using Internet Explorer 6 an appropriate user experience, with Comic Sans fonts everywhere, blinking emphasis tags, animated rainbow hr tags, and animated mailbox graphics added to “mailto:” links!
SexyBookmarks adds an attractive social bookmarking menu to your posts, pages, index, or any combination of the three.
TagSpace lets visitors browse your WordPress categories and tags by flying through them in 3D.
WP Maintenance Mode adds a maintenance-page to your blog that lets visitors know your blog is down for maintenance. Users with rights for theme-options get full access to the blog.