MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
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
tSIP - tomeko.net/software: SIP client for Windows.
YakYak - Desktop client for Google Hangouts