Flock is a “social browser” based on the Firefox 3 code base with additional support for Web 2.0 baked into it. That is, it has sidebars for social networking sites, detects media streams, comes with support for blogging (this post is being done from inside the Flock tool), and more.
It does the job pretty well, but it feels overly heavy. The UI is really busy and sometimes arcane. I’m not sure what it gains me other than avoiding installing some Firefox add-ons that I’ve already grown to know and love.