XO Hosted PBX - Hosted PBX is a scalable UC solution that uses cloud technology to integrate HD voice & data services, mobility, along with advanced calling
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Sipdroid - Free and open-source SIP/VoIP Client
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...