Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Sipdroid - Free and open-source SIP/VoIP Client
tSIP - tomeko.net/software: SIP client for Windows.