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'.
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.