Smooch.io for Facebook Messenger - Message your customers from Slack, Front, a bot or anywhere!
GetChat.App - Add WhatsApp chat to your website for free
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Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Avochato - Text messaging for your business.
Sparkcentral - Good old fashioned customer service, delivered in a high tech way