Smooch.io for Facebook Messenger - Message your customers from Slack, Front, a bot or anywhere!
Avochato - Text messaging for your business.
Message Me - Website to create your contact business card
Talkus - Chat with your website users from Slack
StackWhats - Add the WhatsApp Live Chat to your website
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.