Tinode - Tinode Instant Messaging
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.
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.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
WowApp - Wowapp is the best alternative to Skype and you will get paid to use it!
Mumble - Mumble is an open source, low-latency, high quality voice chat software primarily intended for use...