VoIP - SIP and RTP stacks, softphones, user agents, STUN

SIP Stacks

Probably the most used SIP stacks are (in alphabetical order): osip2/eXosip2, pjsip, resiprocate, sofia-sip

dissipate: C++; Linux, requries the qt-library, License: GPL; Homepage: http://www.div8.net/dissipate/; The original dissipate by Billy Biggs. No more active development!
dissipate2: C++; Linux, requries the qt-library, License: GPL; Homepage: http://www.wirlab.net/kphone/; An enhanced dissipate, is part of the kphone distribution. The stack is strongly tight to kphone, thus can't be used as standalone.
GNU osip(2): C; Linux+Windows+...; License: LGPL; Homepage: http://www.gnu.org/software/osip/; Also known as libosip. Note: The interface of osip has been changed and from now on it will be called osip2! Download the tar file from http://www.antisip.com/download/. Here you can find the API documentation.
GNU eXosip(2): C; Linux+Windows+...; License: GPL; Homepage: http://savannah.nongnu.org/projects/exosip/; The extensible osip: "...It aims to implement a simple high layer API to control the SIP for sessions establishements and common extensions. Once completed, this eXtended library should provide an API for call management, messaging and presence features.... Download the tar file from http://www.antisip.com/download/. Here you can find the API documentation.
SIP from vovida.org: C++; Linux+Windows+...; License: Vovida Software License; Homepage: http://www.vovida.org/protocols/downloads/sip/. No more active development.
resiprocate: A multi-plattform object oriented SIP stack with a very liberal license continuous development. Also includes now a high level call API (DUM=DialogUsageManager) which supports REFER, NOTIFY, TLS, DTLS ... License: VOCAL; Homepage: http://www.resiprocate.org.

Microsoft RTC API: The Mircosoft RTC API is a high level SIP and RTP Stack. It's included in Windows XP and also comes with Windows Messenger. Version 1.2 introduced a lot of new features is behaves strange when used with other SIP clients. Meanwhile the RTC API was replaced by the Microsoft Unified Communications Client.
sipXtackLib: Part of pingtel's open source releases for VoIP. License: LGPL; Homepage: sipfoundry.org. There is also a high level call library (sipXcallLib), which implements JTAPI in C++.
libmsip: A C++ SIP stack for Linux developed for the miniSIP (TLS SIP client) project. Homepage: http://www.minisip.org/libmsip/.
sofia-sip: A cross plattform (Windows/Linux/OSX) SIP stack written in C. Support for STUN, SIMPLE. License: LGPL Homepage: http://sofia-sip.sourceforge.net/.
phApi (phone API): A phone API based on eXosip, ortp. Supports STUN. Part of the openwengo project. Homepage: http://dev.openwengo.com/trac/openwengo/trac.cgi/wiki/PhApi.
PJSIP: Leightweight, but fully complete and highly protable SIP stack with additional media libraries. License: LGPL; Homepage:http://www.pjsip.org/.
OpenSipStack: A open source SIP stack for Windows and Linux. License: MPL1.0. Homepage:http://www.opensourcesip.org/.
sipstack: A GPL SIP stack for Symbian S60 plattform. Project Website:https://forge.betavine.net/projects/sipstack/.
libsip: A SIP stack provided by Sun to be included into Open Solaris. License is Open Source (CDDL). Website:http://playground.sun.com/sip/.
Konnetic Unify SIP .NET SDK: A commercial .NET SIP stack, running on Windows and Mono. Website: www.konnetic.com.