Asterisk: Open Source telephony switching and private branch exchange (PBX) daemon. Supported signalling protocols: H.323, SIP, MGCP, SCCP (Cisco Skinny).
Ekiga: Videoconferencing application to make audio and video calls to remote users. Supports both SIP and H.323.
Gnome-o-Phone: Graphical VoIP software for the Linux Gnome desktop. Includes RtpTunnel utility for tunneling RTP UDP-based traffic through a firewall.
GNU Bayonne: A free, scalable telecommunications application server by the GNU Project.
GNU oSIP: Implementation of Session Initiation Protocol (SIP). This library provides an interface to initiate and control SIP based sessions.
isdn2h323: Project homepage for isdn2h323, a Linux-based ISDN to H.323 gateway.
KPhone: SIP based VoIP phone. It supports Presence and Instant Messaging.
Linphone: SIP based web phone. Supported audio codecs are G711, LPC10-15, GSM, and SPEEX.
Minisip: SIP based phone. Supports video, communication encryption and push-to-talk.
NAT H323 under Linux: Mini-HOWTO for NAT & H.323 - configuring Linux to support H.323 while using network address translation.
Nautilus Secure Phone: Program that allows two parties to hold a secure voice conversation using Linux and TCP/IP.
OpenH323 Gatekeeper: Free H.323 gatekeeper (GPL) based on OpenH323.
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.
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.
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).
SIPSet: Sip based phone. You can use it as a soft phone, to make and receive calls from your Linux PC.
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.
Vovida: VOCAL is an open source project dealing with VoIP. It includes a SIP based Redirect Server, Feature Server, Provisioning Server and Marshal Proxy.