Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
tSIP - tomeko.net/software: SIP client for Windows.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
PBXware - PBXware is the world’s first and most mature IP PBX Professional Open Standards Turnkey Telephony Platform. Bicom Systems’ softswitch is offered in the Business, Call Center and Multi-Tenant Editions each supporting specific features.
Sipdroid - Free and open-source SIP/VoIP Client
CommPeak PBX - CommPeak’s Hosted Cloud IP-PBX is a web-based highly customizable and feature-rich solution - the ideal, low-cost solution for an enterprise of all sizes.