WebRTC uses UDP holepunching via STUN, it's built into almost all browsers and there are free STUN servers online. If you have some types of more tricky network

WebRTC uses UDP holepunching via STUN, it's built into almost all browsers and there are free STUN servers online. If you have some types of more tricky networks (usually symmetric NAT or more tricky firewalls) you need TURN which basically proxies the encrypted stream over TCP via the TURN server, but most home and smaller networks can do holepunching. You still need signaling of the SDP offer, but that can be done via email/sms/whatever if you really don't want to have a server at all. This service is just run-of-the-mill WebRTC, nothing special about it though.