Asterisk

Asterisk is an Open Source with excellent VoIP/ SIP features.

• Asterisk@home - pre-configured Linux distro with most of the essentials.
• astmasters.net - Mac OS X resources (more under VoIP)
• Fedora Core 3 RPMs
• Configuration walkthrough
• Installing from source on Mac OS X (outdated, but with a few useful tidbits)

APIs

• Asterisk PHP, phpAGI - PHP libraries to control Asterisk

Front-Ends (more)

• destar (Python)
• AMP - Integrates a few other interesting tools.

• Implementing proximity detection and call redirection.

The source code tarball for version 1.2.7.1 builds without any issues in Mac OS X 10.4.6 (PowerPC)