SIP2SIP
SIP2SIP is a multimedia real-time communications service based on SIP, WebRTC and related protocols. The service is free to use based on a fair-use policy and federates with other publicly reachable SIP and XMPP domains.
Features
SIP and Web addresses
SIP to XMPP gateway
SIP to WebRTC gateway
Multi-party conferencing
To sign-up go to http://sip2sip.info