H.323 is a VoIP signaling protocol that predates SIP by five or six years, but its use in commercial telephony and desktop conferencing apps (NetMeeting, for instance) is widespread.
OpenH323 is an open source implementation of the H.323 signaling protocol suite, managed by Quicknet Technologies, the same company that makes the Internet Phone Jack line of analog interface cards. OpenH323 is distributed in binary and source code forms for both Linux and Windows, though a crafty hacker should be able to get it running on a BSD-ish OS, too.
This project will allow a Microsoft NetMeeting H.323 softphone and an OpenH323 OhPhone softphone to place calls through an H.323 gatekeeper running on a Linux computer. In this example, I'll use Microsoft NetMeeting on Windows XP and OhPhone on Mac OS X.
Although OpenH323 provides a framework of tools for developing H.323 servers and endpoints, it also natively implements a complete H.323 gateway, MCU, and endpoint. Here's a partial list of software packages that accompany OpenH323:
A simple H.323 gatekeeper server example
An H.323 softphone for Linux and Windows (OhPhoneX is the Macintosh version)
An H.323 conference bridge server
An H.323 gateway server
Each requires the base distributions of OpenH323 and its prerequisite, PWLib, a project-specific class library.
A Pentium III clocked at 600 MHz should be sufficient to handle the role of a small-scale H.323 ...