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

SIP Applications (Proxy, PBX, ...)

Sip Express Router (ser): GPL SIP proxy with a lot of features and a still ongoing development. Homepage: http://www.iptel.org/ser/. You can also take a look at the development homepage with web CVS. There is effort for a new release called "Ottendorf" with overhaul of the core with great new features. Nevertheless it still misses (like stable version) documentation.
Openser: A fork of the Sip Express Router with heavy development and lots of new features. Development is more community driven and it has a great and complete documentation (compared to ser). Homepage: http://openser.org/.
Ser Media Server (sems): Media Server add-on for ser SIP proxy. Homepage: http://developer.berlios.de/projects/sems. Supports voicemail, IVR, SIP/PSTN gateway (supported cards) ...
Asterisk: Linux Software PBX with Gateway, SIP Proxy, Gateway (SIP, H.323, PSTN, ...); Can act as B2BUA, Voicemailserver, ... highly flexible. Homepage: http://www.asterisk.org/
Callweaver: A fork of Asterisk (formerly known as OpenPBX); Includes channels and applications which are not in stock Asterisk release due to missing disclaimer; also supports T.38; Homepage: http://callweaver.org/
partysip: A Linux SIP proxy based on osip2 (LGPL). Development stalled. Developer homepage is at: http://savannah.nongnu.org/projects/partysip/, you can download tar packages from: http://www.antisip.com/download/.
Bayonne: A Linux Telephony server which supports SIP and H.323. Includes a SIP proxy/registrar. Homepage: http://wiki.gnutelephony.org/index.php/GNU_Bayonne.

SIP TAPI: A TAPI driver for MS Windows. Allows to make SIP calls from Outlook or any other TAPI application. Based on AstTapi and eXosip2. GPL. Homepage: http://www.enum.at/index.php?id=479. A commercial version with multiline support (suitable for Citrix environments) and signaling of incoming calls is available too.
Fomine RTC server: A SIP proxy server for Windows which uses its own SIP stack (does NOT need the RTC API) Homepage: http://www.fomine.com/rtc-server.html. The unregistered version can be used up to 5 users.
sipXpbx: Part of pingtel's open source releases for VoIP. License: LGPL; Homepage: sipfoundry.org. This PBX combines various sipX applications like a SIP proxy (sipXregistry, sipXproxy), a media server (sipXvxml) and lots more.
yate: Yet Another Telephony Engine - a PSTN gateway. License: GPL; Homepage: yate.null.ro. This gateway supports H.323, SIP and zaptel (->asterisk) based PSTN cards.
yxa: YXA is a SIP proxy written in Erlang. License: BSD; Homepage: http://www.stacken.kth.se/projekt/yxa/index.html.
Freeswitch: Freeswitch is a multi protocol (SIP, IAX, jingle ...) softswitch written in C. Runs on Linux, BSD, Windows ... Homepage:http://www.freeswitch.org/.

OpenSBC: A SBC/B2BUA based on the OpenSipStack. Runs on Linux and Windows. License: MPL1.0. Homepage:http://www.opensourcesip.org/opensbc.php.

3CX Phone System: A commercial SIP proxy/PBX which runs under Windows. They also offer a free version of their 3CX phone system. Homepage:http://www.3cx.com/.

SIPWright Proxy: A commercial SIP proxy. A free version is available at http://www.estacado.net/SIPWright.html.

TERASIP: A commercial TAPI service provider for SIP. A free version is available at http://www.terasens.com/products/terasip/default.aspx.

Sippy: Sippysoft (portaone) released its Sippy B2BUA as GPL. It is written in Python. See http://www.b2bua.org/ and devloper homepage at http://developer.berlios.de/projects/sippy/.

SIPSorcery: SIPSorcery is an online call management application, allowing you using multiple SIP providers services with one piece of hardware. See https://www.sipsorcery.com/ and devloper homepage at http://sipsorcery.codeplex.com/.

NetSIPServer: NetSIPServer is a SIP server programmed in C# with GPL license. See http://netsipserver.codeplex.com/.

CallButler: CallButler is a Windows based PBX. See http://callbutler.codeplex.com/. Seems inactive now.