Anologue - Disposable online chat.
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.
Shtuff Chat - Free Group / Team chat with persistent history.
Chatib - Chatib is an online chat room where you can meet and chat with new people, make them friends, or even data them online.
Tlk2me - Tlk2me is simple real-time messaging platform which is designed exclusively for groups.
Chattusa - Chattusa is a social network website that provides a free chat service and lets you connect with people from the USA without any hassle.