Smooch.io for Facebook Messenger - Message your customers from Slack, Front, a bot or anywhere!
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
GetChat.App - Add WhatsApp chat to your website for free
Carrd - Simple, responsive one-page site creator.
Avochato - Text messaging for your business.
Talkus - Chat with your website users from Slack