Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
GlobalMeet Collaboration - From cloud PBX to conference calls and meetings to online events, GlobalMeet is a business communications platform that helps take you where you need to go.
Sipdroid - Free and open-source SIP/VoIP Client
MiCloud Connect - Secure and flexible cloud phone services and unified communications.