Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
YakYak - Desktop client for Google Hangouts