I'm writing this as an attempt to make campus transfer suck significantly less -- currently there are about eleventy billion WLANs and none of them can auto-discover the other ones through Bonjour. This makes it somewhat hard for me to hear other people's iTunes shares, among other things.
So I started writing on this thing. With a centralized server, it registers the various addresses and what they offer for download -- then you click on it, pick "Download" and the server tells you where to pick it up.
Right now, that's as far as I've gotten -- but the client is intended to pick up on that clue, contact the one that belongs to the person holding the file, and go "Bring me that file!".
The protocol is more or less plain text, so I'm planning on also offering a command line leeching utility for downloading things from shares once you have telnetted into the tracker and done the necessary voodoo.