Quassel - Open Source IRC Client

Quassel IRC, or Quassel, is a graphical, distributed, cross-platform IRC client, introduced in 2008. It is released under the GNU General Public License for Linux and Unix-like operating systems, OS X, and Microsoft Windows. Since the release of Kubuntu 9.04 (Jaunty Jackalope) Quassel is Kubuntu's default IRC client.

Twinkle - Open Source VoIP Solutions

Open Source VoIP Application

Twinkle is a open source software application for Voice over Internet Protocol (VoIP) voice communications in IP networks, such as the Internet. It is designed for GNU/Linux operating systems and uses the Qt toolkit for its graphical user interface. For call signaling it employs the Session Initiation Protocol (SIP). It also features direct IP-to-IP calls. Media streams are transmitted via the Real-time Transport Protocol (RTP) which may be encrypted with the Secure Real-time Transport Protocol (SRTP) and the ZRTP security protocols.

Ekiga – Open Source VoIP Service

Open Source VoIP Client for Windows & Linux

Ekiga is an open-source VoIP service that is compatible with Windows and Linux. Ekiga allows you to make voice and video phone calls and to send text messages to cell phones. It also enables standard telephony features like call holding, call forwarding and dial tone.
Ekiga supports a common VoIP protocol called session interface protocol and offers you a free SIP address during registration. You can configure multiple SIP addresses through the client and use those addresses simultaneously. Ekiga does not support encryption.

QuteCom - Open Source Softphone

Open Source Communication Software

QuteCom is a SIP compliant software. VoIP client developed by the QuteCom (previously OpenWengo) community under the GNU General Public License (GPL). It allows users to speak to other users of SIP compliant VoIP software at no cost. It also allows users to call landlines and cell phones, send SMS and make video calls. QuteCom is an free and open source voice-over-ip, and video conferencing application. The latest version of the software is always available on the QuteCom website.

QuteCom 2.2 is licensed under the GPL, and is free software.