Dialogflow - Conversational UX Platform. (ex API.ai)
Hubot - Hubot is a standardized way to share scripts between everyone's robots.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
opsdroid - An open source chat bot framework written in python.
Botpress - Open-source platform for developers to build high-quality digital assistants
Expensive Chat - Chatroom where you pay per letter