SMSGlobal - SMSGlobal offers a web-based platform and API's enabling businesses to send mass SMS's.
Avochato - Text messaging for your business.
Tatango - Tatango is a mobile messaging platform for agencies.
Postscript - Like Mailchimp for SMS
GetChat.App - Add WhatsApp chat to your website for free
Flowroute - Flowroute provides direct access to telephony resources - such as calling, messaging (SMS & MMS), call routing, SIP Trunking and Communication APIs.