Reliable UDP implementation for Node.js.
For a peer-to-peer application, you can write a script like this:
var rudp = ;var dgram = ;var socket = dgram;socket;var client = socket remoteAddress remotePort;// And do whatever you want here
No really; that above code will actually work, even if all hosts were sitting behind a NAT. Give it a try. For the heck of it, give rudp a try with a file sharing example.