Zoiper runs on a multitude of different platforms: Mac, Linux or Windows, iPhone and Android - with support for both SIP and IAX, and includes free and paid versions of their software.
Microsip is free open source SIP softphone that runs on Windows OS, and is also portable.
Switchvox Softphone app gives you all of the enterprise-class Unified Communications features you expect from Switchvox, now available on the go.
The 3CX softphone for Windows is a free softphone that you can use to make and receive VoIP phone calls from your PC. The advantage of using the 3CX softphone for Windows is that you can leverage low cost or free VoIP calls.
Bria Solo (the new X-lite Softphone) combines support for multiple SIP accounts with team communication and collaboration services from CounterPath supporting a range of codecs and also offering great support for desktop business video Microsoft Teams.