VoIP.ms - VoIP.ms is a VoIP origination and termination tool.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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
Loop Communications - Loop Communications provides hosted VoIP business phone systems to small businesses and mid-sized companies.
Sipdroid - Free and open-source SIP/VoIP Client