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
Mitel - Business Communications and Collaboration
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Eir SIP Voice - eir SIP Voice is revolutionizing the core communications tool of business that carries both calls and data over just one connection.
Sipdroid - Free and open-source SIP/VoIP Client
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...