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

Twilio - Brings voice and messaging to your web and mobile applications.

Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.

Plivo - Plivo simplifies your customer engagement.

LiveChat - LiveChat - premium live chat software. Approach customers directly on the website, make connections and drive more sales using LiveChat.

Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates