KPhone
- SIP based VoIP user agent. It supports Presence and Instant Messaging.
- http://sourceforge.net/projects/kphone

Linphone
- SIP based web phone. Supported audio codecs are G711, LPC10-15, GSM, and SPEEX.
- http://www.linphone.org/

Skype
- Peer to peer voice service. Users may call landlines and cellphones for a fee; users may call each other for free. Source code is not available.
- http://www.skype.com/

Siproxd Project
- Proxy/masquerading daemon for the SIP protocol. It handles registrations of SIP clients on a private IP network and performs rewriting of the SIP message bodies to make SIP connections work via an masquerading firewall (NAT).
- http://siproxd.sourceforge.net/

Vovida
- VOCAL is an open source project dealing with VoIP. It includes a SIP based Redirect Server, Feature Server, Provisioning Server and Marshal Proxy.
- http://www.vovida.org/

Asterisk
- Open Source telephony switching and private branch exchange (PBX) daemon. Supported signalling protocols: H.323, SIP, MGCP, SCCP (Cisco Skinny).
- http://www.asterisk.org/

GNU Bayonne
- A free, scalable telecommunications application server by the GNU Project.
- http://www.gnu.org/software/bayonne/bayonne.html

Ekiga
- Videoconferencing application to make audio and video calls to remote users. Supports both SIP and H.323.
- http://www.gnomemeeting.org/

isdn2h323
- Project homepage for isdn2h323, a Linux-based ISDN to H.323 gateway.
- http://www.telos.de/linux/H323/

GNU oSIP
- Implementation of Session Initiation Protocol (SIP). This library provides an interface to initiate and control SIP based sessions.
- http://www.gnu.org/software/osip/osip.html

SIPSet
- Sip based phone. You can use it as a soft phone, to make and receive calls from your Linux PC.
- http://www.vovida.org/applications/downloads/sipset/

Minisip
- SIP based phone. Supports video, communication encryption and push-to-talk.
- http://www.minisip.org/

Partysip
- SIP proxy server. It can operate as registrar server, redirect server and stateful proxy server. SIP is an open standard (IETF) replacement for H323.
- http://www.nongnu.org/partysip/partysip.html

Nautilus Secure Phone
- Program that allows two parties to hold a secure voice conversation using Linux and TCP/IP.
- http://nautilus.berlios.de/

OpenH323 Gatekeeper
- Free H.323 gatekeeper (GPL) based on OpenH323.
- http://www.gnugk.org/

OpenH323 Project
- The OpenH323 project aims to create a full featured, interoperable, Open Source implementation of the ITU H.323 teleconferencing protocol that can be used by personal developers and commercial users without charge.
- http://www.openh323.org/

Gnome-o-Phone
- Graphical VoIP software for the Linux Gnome desktop. Includes RtpTunnel utility for tunneling RTP UDP-based traffic through a firewall.
- http://gphone.sourceforge.net/