ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
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.
Zulip - Chat for distributed teams. Zulip combines the immediacy of real-time chat with an email threading model. With Zulip, you can catch up on important conversations while ignoring irrelevant ones.
Otr.to - A secure peer-to-peer chat in your web browser.
WowApp - Wowapp is the best alternative to Skype and you will get paid to use it!