Phono

Phono – jQuery Phone Plugin: This is ridiculously cool.  Works great too.  Call the default number that’s in the demo box on page load – I dare you.

Phono is a simple jQuery plugin and JavaScript library that turns any web browser into a phone; capable of making phone calls and sending instant messages. You can even connect to SIP clients; all with a simple unified API.

From the good folks at Voxeo.