Smooch.io for Facebook Messenger - Message your customers from Slack, Front, a bot or anywhere!
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Pullstring - Powering computer conversation across all platforms.
BuildFire - BuildFire is the easiest way for small businesses to build a mobile app in a matter of minutes for iOS and Android.
Conversity - Easiest way to manage Facebook Messenger chats
MIT App Inventor - App Inventor is a cloud-based tool, which means you can create apps for phones or tablets right in your web browser.