Avochato - Text messaging for your business.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Postscript - Like Mailchimp for SMS
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
GetChat.App - Add WhatsApp chat to your website for free
Plivo - Plivo simplifies your customer engagement.