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
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Sipdroid - Free and open-source SIP/VoIP Client
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).