Communifire - Enterprise Social Collaboration Software
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Workplace by Facebook - Connect everyone in your company and turn ideas into action.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
DigitalChalk - Online Training Software and Learning Management System (LMS)
tSIP - tomeko.net/software: SIP client for Windows.