OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
PushBots - Light SDK for mobile push notifications
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
iZooto - Engage your mobile and desktop web users with intelligent web push notifications.
Plivo - Plivo simplifies your customer engagement.