GetChat.App - Add WhatsApp chat to your website for free
Smooch.io for Facebook Messenger - Message your customers from Slack, Front, a bot or anywhere!
It's Alive - Create a Facebook bot in minutes
Avochato - Text messaging for your business.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Talkus - Chat with your website users from Slack