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. 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. OpenH323 Gatekeeper - Free H.323 gatekeeper (GPL) based on OpenH323. 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. GNU oSIP - Implementation of Session Initiation Protocol (SIP). This library provides an interface to initiate and control SIP based sessions. GNU Bayonne - A free, scalable telecommunications application server by the GNU Project. 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 a masquerading firewall (NAT). Linphone - SIP based web phone. Supported audio codecs are G711, LPC10-15, GSM, and SPEEX. KPhone - SIP based VoIP user agent. It supports Presence and Instant Messaging.