Open source softswitch

Freeswitch is a brand new initiative led by a former Asterisk developer.
It is rather a softswitch than a PBX whose main interest lies in its modularity, scalability and integration capabilities : it can provide in-process and out-process call control in a variety of programming languages. It can be compiled & executed both on Linux and Windows platforms.

The first release is expected shortly. In the meantime, you can read the story behind Freeswitch.