Dialogflow - Conversational UX Platform. (ex API.ai)
MessageBird Chat API - Connect with your customers through different messaging apps
Octane AI - Octane AI offers tools to create a bot and engage customers and audience via messaging.
Wavecell - Wavecell simplifies real-time cloud communications using SMS, Chat Apps, Video Interaction and Voice solutions.
Botkit - Botkit is an open source developer tool for building chat bots, apps and custom integrations for major messaging platforms.
Twilio - Brings voice and messaging to your web and mobile applications.