VOIP

Twinkle is a softphone for your voice over IP and instant messaging communications using the SIP protocol. You can use it for direct IP phone to IP phone communication or in a network using a SIP proxy to route your calls and messages.

TeamSpeak is non-free Voice over IP software that allows users to speak on a chat channel with other users, much like a telephone conference call. A TeamSpeak user will often wear a headset with an integrated microphone. Users use the TeamSpeak client software to connect to a TeamSpeak server of their choice, from there they can join chat channels.

Linphone is an internet phone or Voice Over IP phone (VoIP).

* With linphone you can communicate with people using voice, video, and instant messaging.
* Linphone complies to the SIP protocol, an open standard for internet telephony
* If you subscribe a VoIP to PSTN (=classic telephony) you can reach "classic" phone lines

Kphone is a Session Initiation Protocol (SIP) user agent for Linux, which allows Voice over IP (VoIP) connections over the Internet.

Kphone supports Presence and Instant Messaging, and to some extent video calls
between two hosts.

Ekiga (formely known as GnomeMeeting) is a VoIP and video conferencing application. Ekiga uses both the H.323 and SIP protocols. It supports many audio and video codecs, and is interoperable with other SIP compliant software and also with Microsoft NetMeeting.