Dialogflow - Conversational UX Platform. (ex API.ai)
ManyChat - ManyChat lets you create a Facebook Messenger bot for marketing, sales and support.
Botpress - Open-source platform for developers to build high-quality digital assistants
Chatfuel - Chatfuel is the best bot platform for creating an AI chatbot on Facebook.
Messenger Platform - Discovery, chat extensions, and richer experiences
Landbot - An intuitive no-code conversational apps builder that combines the benefits of conversational interface with rich UI elements.