WebRTC is an extension to cutting-edge browsers that allows for P2P connections. A popular example is Google’s AppRTC which sets up a chat between two browser windows. Another interesting example is Sharefest which allows drag and drop file sharing.
Even though the connections for these programs are between two browser windows, connection setup requires accessing servers at known locations on the internet. This intermediating server is known as a signaling server. It communicates SDP and ICE messages back and forth to establish peer connections.