BackWPup allows you to backup your WordPress database, files, and more.
Facebook makes your site deeply social by integrating functionality from Facebook.
Posterous Importer imports posts, comments, tags, and attachments from a Posterous.com blog.
SyntaxHighlighter Evolved allows you to easily post syntax-highlighted code to your site without having to escape the code or anything.
Watu allows you to create exams with unlimited questions and answers, and define grades based on collected points.