Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Facebook Messenger - Facebook Messenger is a faster way to message.
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Digsby - Tagged makes it easy to meet and socialize with new people through games, shared interests, friend suggestions, browsing profiles, and much more.
tSIP - tomeko.net/software: SIP client for Windows.