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...
CSipSimple - CSipSimple is a SIP application for android devices.
Mitel - Business Communications and Collaboration
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.