Hubot - Hubot is a standardized way to share scripts between everyone's robots.
Aivo - Skyrocket your Customer Service and Sales KPIs with a Chatbot powered by Artificial Intelligence. Give time back to people.
opsdroid - An open source chat bot framework written in python.
Zendesk Answer Bot - Chatbots
Dialogflow - Conversational UX Platform. (ex API.ai)
Tars - TARS enables users to create chatbots that replaces regular old webforms.