Unified Communications Server
Unified Communications System - PBX System
Java VoIP Softphone (SIP) (replaced by jfPhone)
Open source streamlined SIP client built on Linphone
Baresip is a modular SIP User-Agent with audio and video support
Optimized and cross platform SMPTE 2022 FEC library in C, Python, Java
The OZEKI VoIP Service Checker allows you to check VoIP services.
C++ SIP stack based on Chrome source code