Via Dave Winer, I found this post by Mike Arrington talking about the AllPeers Firefox extension:
AllPeers is a simple, persistent buddy list in the browser. Initially, interaction with those buddies will be limited to discovering and sharing files - If you choose to, you can share any file on your network with one or more of your friends. They will be able to see what files you choose to share (even getting an RSS feed of new files you include), and with a single click download it to their own hard drive.
AllPeers is private, P2P file sharing using BitTorrent to do the transfer, implemented as a Firefox extension. It involves buddylists as well, so I left a comment on Mike's site that they should use XMPP to power the buddylist. Buddylists and IM capabilities in the browser (which are proxies for identity and peer-to-peer communication, respectively) are destined to have some sort of representation directly in the browser. This might be something for Flock to think about.