Quite a few depending on what level of control/functionality you are looking for.

Some are more developer/write your own code focussed, other more graphically focused.

In alphabetical order and not exhaustive.

API.AI (now Google) Chatfuel (YC) Converse.AI (disclaimer, $currentstartup) Flowxo Gupshup.io IBM Watson Engagement Advisor/Watson Meya.ai Microsoft Bot builder Motion.ai Wit.ai (Facebook)

AWS also has a suite of tools that between them you could use, check out Lex, Polly & Steps

All of these do one or more of the following:

NLP User management Messaging Platform Integrations Conversation State & Context Management Workflow Human Escalation Backend integrations (Helpdesk, CRM etc) Analytics

Pick your poison based on what you need :)