CommentPress is an open source theme for the WordPress blogging engine that allows readers to comment paragraph by paragraph in the margins of a text. Annotate, gloss, workshop, debate: with CommentPress you can do all of these things on a finer-grained level, turning a document into a conversation. It can be applied to a fixed document (paper/essay/book etc.) or to a running blog.
WordTwit is a plugin that utilizes the Twitter API to automatically push a published post to your Twitter account as a tweet, using the blog domain as a short URL.
BuddyPress is a suite of WordPress plugins and themes which extends WordPress MU and brings social networking features to a new or existing installation.
Twitter Tools This plugin creates an integration between your WordPress blog and your Twitter account. Pull your tweets into your blog and create new tweets on blog posts and from within WordPress.
WhoLinks2Me.com is an incredibly useful tool for finding out who links to your website or blog. With Snap Preview Anywhere, you can not only read who links to you, but see them as well!