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.
JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website
Postscript - Like Mailchimp for SMS
tawk.to - tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page
GetChat.App - Add WhatsApp chat to your website for free