Yes VoIP Billing
free opensource billing system for GnuGK

Communications::Internet Phone
Communications::Telephony

TeamSpeakAdmXMP
TeamSpeakAdmXMP is a TeamSpeak server administration tool for use with the game Unreal 2 XMP. It provides an easy to use

Communications::Internet Phone
Games/Entertainment::First Person Shooters
Security
System::Systems Administration
Terminals::Telnet

GroupVoice
A library to provide team-based voice communication over the net, meant for easy integration with multiplayer games

Communications::Conferencing
Communications::Internet Phone
Multimedia::Sound/Audio

OpenMC : Open Source Multimedia Center
OpenMC : The Open Source Multimedia Center for your Television An HTPC / VCR / PVR / TV / Jukenbox Plug In driven Application

Communications::Chat
Communications::Email::Email Clients (MUA)
Communications::Internet Phone
Multimedia::Sound/Audio
Multimedia::Video

simpleRADIUS
simpleRADIUS is the smallest VoIP billing server on earth. it is built on completely independent cross-platform library

Communications::Internet Phone
Communications::Telephony
Internet
Software Development
System

tButils
Utilities for managing a theBridge conference server remotely

Communications::Ham Radio
Communications::Internet Phone

Asterisk GUI client
This software suite is designed to extend the functionality of the Asterisk PBX through platform-independant web-client

Communications::Internet Phone
Communications::Telephony

Shtoom - a python SIP framework & phone
A software SIP (Voice over IP) Framework, including a phone, written in Python

Communications::Internet Phone
Internet

circum
By pushing raw MP3 frames over UDP, what is the lowest latency possible? This is an experimental IP conferencing tool using

Communications::Conferencing
Communications::Internet Phone

SIPSec
A secure SIP VoIP client using the new security features from RFC3261 like S/MIME. A console application written in c

Communications::Internet Phone