Jami (formerly GNU Ring, SFLphone) positions itself as a potential free Skype replacement
Linphone is one of the most famous open source softphones in the world. It is fully SIP-based, for all calling, presence and IM features.
MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack. It allowing to do high quality VoIP calls (P2P or on regular telephones) via open SIP protocol. From cloud of SIP providers you can choose best for you, register account and use it with MicroSIP. You'.
LINE is a Japanese proprietary application for instant messaging on smartphones and PCs. Line users exchange text messages, graphics, video and audio media, make free VoIP calls, and hold free audio or video conferences.
Psi is a cross-platform powerful XMPP client designed for experienced users. Psi+ is a development branch of Psi with rolling release development model.
Users who wants to receive new features and bug fixes very quickly may use Psi+ on daily basis. Users who do not care about ne.
Zoiper IAX & SIP multilanguage and multiplatform (Windows, Linux and Mac OS X) softphone is a VoIP soft client, meant to work with any IP-based communications systems and infrastructure.
PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP). You need an internet connection and an account from a provider supporting the SIP protocol.
PhonerLite vs Bria opinions
Very small, simple and useful solution. It may lack a few festures, but it is robust and allow to make call rather easily
Blink is the best real-time communications client using the SIP protocol.
Blink vs Bria opinions
best free alternative, most features included
Telephone allows you to make phone calls over the Internet or your company network. If your phone line supports SIP protocol, you can use it on your Mac instead of a physical phone anywhere you have a decent network connection.