Dialogflow - Conversational UX Platform. (ex API.ai)
Alexa Skill Kit - Effortless Alexa Skill development with AWS Lambda
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
BotTalk - Create Alexa skills and Google Assistant actions with Markup
Botpress - Open-source platform for developers to build high-quality digital assistants
Actions on Google - The open platform for Google Assistant