Hallo quantumgravity,There are dozens of websites related to WebRTC but they are pretty much all dealing with the same technology. [1]
AFAIK WebRTC is completely free.I remember seeing a website recently with "Call me" buttons for Google, Twitter, etc... but I can't find it anymore unfortunately. Maybe this [2] goes in the right direction, though I'm not sure if this is really free.
[1]: https://en.wikipedia.org/wiki/WebRTC [2]: http://s.phono.com/releases/1.1/samples/callme/index.htm
