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.
Client VoIP for Windows Mobile
C++ SIP stack based on Chrome source code