Voxer - Turn your phone into a Walkie Talkie. Don't waste time on phone calls and voicemails.
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.
Investis - Investis helps companies to use digital channels to increase revenue, enhance employee engagement and influence corporate audiences.
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.
Sameroom - Chat interoperability-as-a-service
WowApp - Wowapp is the best alternative to Skype and you will get paid to use it!