Java VoIP Softphone (SIP) (replaced by jfPhone)
SIP Video Multiconference Media Server with WebRTC support.
Positional Audio Communication for Minecraft with Mumble
Open source streamlined SIP client built on Linphone
Optimized and cross platform SMPTE 2022 FEC library in C, Python, Java
The OZEKI VoIP Service Checker allows you to check VoIP services.