OpenWengo is a community of enthusiasts and developers, creating free software products related to communication over IP. The flagship product of the OpenWengo project is a softphone which allows you to make free PC to PC video and voice calls, and to integrate all your IM contacts in one place.
iaxComm is cross-platform program developed on aWindows XP system.
iaxComm is targeted for the Win32, Linux and Mac OS X systems. I try to avoid anything that will cause a problem under Mac OS, but have no way of building or testing on a Mac.
Minisip is a SIP User Agent ("Internet telephone").
It can be used to make phone calls, instant message and videocalls to your buddies connected to the same SIP network.
MjSip is a complete java-based implementation of a SIP stack.
It provides in the same time the API and implementation bound together into the MjSip packages. MjSip is available open source under the terms of the GNU GPL license (General Public Licence) as published by the Free Software Foundation.
Ekiga (formely known as GnomeMeeting) is an open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols. It supports many audio and video codecs, and is interoperable with other SIP compliant software and also with Microsoft NetMeeti
SFLphone is meant to be a robust enterprise-class desktop phone. It is designed with a hundred-calls-a-day receptionist in mind. It can work for you, too.
voip-info: Open Source VOIP Software