Open source PBX.

Asterisk is one of the Open Source PBX software. Which supports on many platforms include GNU/Linux, All BSD unix systems and QNX. It is the package comes in the Debian Linux distribution.

It is having vast features, and supports many of the VoIP protocols include, SIP, H.323, MGCP, SCCP. You can get a clear list of supported features from this link

http://www.asterisk.org/support/features

and more about the Asterisk from the link

http://www.asterisk.org/support/about

Another Open source PBX project is FreeSwitch.

The comparisions with the Asterisk, can get from the link.

http://www.freeswitch.org/node/117

And features from the link

http://wiki.freeswitch.org/wiki/Specsheet

-srinivas.