SuprSend - Single Notification API for Every Channel (SMS, Slack, Whatsapp, Email, SMS, Push, App Inbox)
Avochato - Text messaging for your business.
MagicBell - Your app needs a notification inbox – with real-time sync, web & mobile push, and a UI that matches your brand.
Message Me - Website to create your contact business card
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.
Slack with your Users - Speak to your app's users entirely within Slack