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 oparators you can choose best for you, register account and use it with MicroSIP. You'll get free p2p calls and cheap international calls.
3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such as Google Nexus, Sony Xperia, Motorola Droid or Samsung Galaxy. Connect 3CXPhone to a VoIP oparators or to a VoIP PBX (for example 3CX Phone System) to make calls to any VoIP, mobile or landline number.
Zoiper 5 is a free VoIP phone available for Mac. Connect Zoiper 5 to a VoIP oparators or to a VoIP PBX to make calls to any VoIP, mobile or landline number. The program has been properly developed with a fresh user interface that is simple to use and remains out of the way so you can concentrate on the calls.