MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
tSIP - tomeko.net/software: SIP client for Windows.
We-Talk - Free online chat rooms where people chat and meet each other and chat with the stranger and learn new cultures and make friends.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Phoner - Softphone for VoIP, CAPI und TAPI.