Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Sipdroid - Free and open-source SIP/VoIP Client
Jitsi - Open-source video calling and chat platform.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.