Creating a webRTC peer *without* a browser, with just a JavaScript interpreter

I think you could use a node.js server to do so. There's a npm package bringing webrtc capabilites to nodejs : node-webrtc.

Very late answer, but I think it's good to re-evaluate this question, because a lot has changed since this question was asked.

I assume this question was asked because there was no native support for webrtc yet at the time. But there is now. Android, iOS, Windows, Linux and OSX all support native webrtc libraries now.

The native libraries can be used to create a peerconnection and setup a stream to another client (cross-platform). If you want to create any webrtc-based client application without using a browser, the native libraries are the way to go. No silly standalone javascript engine necessary.

Read more here

You could do this with headless chrome. Chrome of course has full WebRTC support, but can be started in "headless" mode, and then interacted with via the command line or their control interface.