GetChat.App - Add WhatsApp chat to your website for free
SuprSend - Single Notification API for Every Channel (SMS, Slack, Whatsapp, Email, SMS, Push, App Inbox)
Smooch.io for Facebook Messenger - Message your customers from Slack, Front, a bot or anywhere!
MagicBell - Your app needs a notification inbox – with real-time sync, web & mobile push, and a UI that matches your brand.
Avochato - Text messaging for your business.
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.