Blink is an SIP client distributed under the Blink license (GNU GPLv3 with the exception of possibility to include Proprietary software modules for commercial purpose) 
The software is written in Python for Cocoa. and lately it was ported to Qt for supporting Microsoft Windows and GNU/Linux platforms.
Blink is based on the Session Initiation Protocol, and beyond voice over IP, its features also include video, instant messaging, file transfer and multi-party conferencing sessions based on MSRP protocol, remote desktop sharing using RFB protocol (VNC), and SIMPLE presence using XCAP protocol.
Privacy and Security
Mac OSX version of Blink implements end-to-end encryption and peer verification for chat media using OTR protocol, audio and video media using ZRTP protocol (all versions).