Zoiper is an easy to use sip video softphone, with excellent voice quality and easy to setup. Developed by ZoIPer
License: Freemium
Apps available for Mac OS X Windows Linux Online Android
MicroSIP is a portable SIP softphone based on the PJSIP stack available for Microsoft Windows.
License: Open Source
Apps available for Windows PortableApps.com
MicroSIP is a lightweight, open-source VoIP softphone focused on simplicity and portability, while Zoiper offers a more comprehensive feature set with a user-friendly interface but is not open-source and comes with some paid features.