Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
We-Talk - Free online chat rooms where people chat and meet each other and chat with the stranger and learn new cultures and make friends.
Otr.to - A secure peer-to-peer chat in your web browser.