Socket.io - Realtime application framework (Node.JS server)
Google App Engine - A powerful platform to build web and mobile apps that scale automatically.
Ably - Serious realtime infrastructure
Dokku - Docker powered mini-Heroku in around 100 lines of Bash
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Google Cloud Functions - A serverless platform for building event-based microservices.