eXosip

eXosip is a new library based on oSIP. It contains a high layer easier to use for implementing SIP End point.

eXosip is a library that hides the complexity of using the SIP protocol for mutlimedia session establishement. This protocol is mainly to be used by VoIP telephony applications (endpoints or conference server) but might be also usefull for any application that wish to establish sessions like multiplayer games.
Free VoIP PBX Software
Asterisk

Asterisk is a complete PBX in software. It runs on Linux and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in three protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.

Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing. It has support for three-way calling, caller ID services, ADSI, SIP and H.323 (as both client and gateway).

Asterisk needs no additional hardware for Voice over IP. For interconnection with digital and analog telephony equipment, Asterisk supports a number of hardware devices, most notably all of the hardware manufactured by Asterisk's sponsors, Digium. Digium has single and quad span T1 and E1 interfaces for interconnection to PRI lines and channel banks as well as a single port FXO card and a one to four-port modular FXS and FXO card.

Also supported are the Internet Line Jack and Internet Phone Jack products from Quicknet.

Asterisk supports a wide range of TDM protocols for the handling and transmission of voice over traditional telephony interfaces. Asterisk supports US and European standard signaling types used in standard business phone systems, allowing it to bridge between next generation voice-data integrated networks and existing infrastructure. Asterisk not only supports traditional phone equipment, it enhances them with additional capabilities.

Using the Inter-Asterisk eXchange (IAX) Voice over IP protocol, Asterisk merges voice and data traffic seamlessly across disparate networks. While using Packet Voice, it is possible to send data such as URL information and images in-line with voice traffic, allowing advanced integration of information.

Asterisk provides a central switching core, with four APIs for modular loading of telephony applications, hardware interfaces, file format handling, and codecs. It allows for transparent switching between all supported interfaces, allowing it to tie together a diverse mixture of telephony systems into a single switching network.

Asterisk is primarily developed on GNU/Linux for x/86. It is known to compile and run on GNU/Linux for PPC along with OpenBSD, FreeBSD, and Mac OS X Jaguar. Other platforms and standards based UNIX-like operating systems should be reasonably easy to port for anyone with the time and requisite skill to do so. Asterisk is available in the testing and unstable Debian archives, maintained thanks to Mark Purcell.
GNU Bayonne

GNU Bayonne, the telephony server of GNU Telephony and the GNU project, offers free, scalable, media independent software environment for development and deployment of telephony solutions for use with current and next generation telephone networks.

GNU Bayonne supports IVR scripting using hardware from Voicetronix, Dialogic, Aculab, CAPI drivers, and Quicklink drivers under GNU/Linux. Bayonne performs script driven IVR applications written in GNU Bayonne's native scripting language, as well as access, conversion, and playing of audio from remote URL's.
FreeSWITCH

FreeSWITCH is an open source telephony application written in C, built from the ground up and designed to take advantage of as many existing software libraries as possible. FreeSWITCH makes it possible to build an open source PBX system or an open source voip switching platform as well as unite various technologies such as SIP, H.323, IAX2, LDAP, Zeroconf, XMPP / Jingle etc. FreeSWITCH can also be used to interface with other open source PBX systems such as Asterisk, GNU Bayonne, or OpenPBX.
OpenPBX

OpenPBX.org is an open Source Private Branch Exchange System (PBX) in software for the Linux Operating system. OpenPBX.org is licenesd under the GNU General Public License or GPL.
Other VoIP Software
fobbit

Fobbit allows Creative VOIP Blaster hardware devices to be used under NetBSD, Linux, and Microsoft Windows. It permits calls to be made to other Fobbit users without the need for the original Creative Labs software, and works from behind firewalls and NAT.
CPhone

CPhone is a cross-platform GUI for the OpenH323 VOIP libraries.
SIPTiger

SIPTiger is a web-based provisioning utility for Cisco's line of 7960 and 7940 Session Initiation Protocol (SIP) IP phones and Cisco SIP Proxy Servers (CSPS). This utility is useful for anyone deploying Cisco 7960/7940 SIP IP Phones.

SIPTiger version 2.3.1 is now available with expanded functionality and several bug fixes. See the readme file for more details.

Cisco 7960/7940 SIP IP phones and Cisco SIP proxy servers are both reliant upon a set of configuration files, which SIPTiger can parse and format into a user-friendly web-based Graphical User Interface (GUI). After these files are modified, the affected SIP phones can then be remotely reloaded to allow the changes to take effect. SIPTiger also supports administrative-level call forwarding configuration.