tSIP - tomeko.net/software: SIP client for Windows.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Acrobits Softphone - Acrobits Softphone, an app for entry into the world of SIP.
Sipdroid - Free and open-source SIP/VoIP Client