SuprSend - Single Notification API for Every Channel (SMS, Slack, Whatsapp, Email, SMS, Push, App Inbox)
SendBird - SendBird is messaging-as-a-service.
MagicBell - Your app needs a notification inbox – with real-time sync, web & mobile push, and a UI that matches your brand.
Chat by Stream - Build and ship in app messaging in a few hours
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.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.