Pusher Beams - Send push notifications programmatically at scale
VisitorEngage - Feedback tab and in-app push notifications for websites based on visitor's behavior on the site.
Truepush - Unleash limitless possibilities with Truepush and save up to $600 monthly. Free up to 10K Subscribers on all features.
Magento 2 Push Notifications Extension - Want to stay in touch with your customers always?
PushOwl - Re-engage website visitors with push notifications
Socket.io - Realtime application framework (Node.JS server)