I almost expect AWS to release their own "Bot Framework" soon.
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 :)
You can use the API to converse with people on web chat, iOS/Android in-app chat, SMS, Facebook, Telegram, Viber, Twitter DM, WeChat, LINE+++ all through one single set of APIs. You can even send rich structured messages that are automatically translated into the best representation on each platform.