Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Tatango - Tatango is a mobile messaging platform for agencies.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Postscript - Like Mailchimp for SMS
Plivo - Plivo simplifies your customer engagement.
Flowroute - Flowroute provides direct access to telephony resources - such as calling, messaging (SMS & MMS), call routing, SIP Trunking and Communication APIs.