Flock is a web browser built on Mozilla’s Firefox codebase that specializes in providing social networking and Web 2.0 facilities built into its user interface.  On June 16th 2008, the latest version of the browser, Flock 2 (based on Mozilla's Firefox 3), went into open beta.
I will admit that it's pretty cool. It allows you to keep up with sites like Facebook, YouTube, and various other sites. In fact, I'm writing this right from it's 'Blog Post' interface!
You can get it here.