Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Lingo - Empower Creation and Ensure Brand Consistency. Build and Share libraries of visual assets with your entire team — all from your desktop.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
tSIP - tomeko.net/software: SIP client for Windows.
Sipdroid - Free and open-source SIP/VoIP Client