Socket.io - Realtime application framework (Node.JS server)
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Ably - Serious realtime infrastructure
restdb.io - restdb.io lets you create databases and REST APIs quickly. Development databases are free. Powerful web-based and mobile-friendly data management.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
RemoteStorage - An open protocol for per-user storage OWN YOUR DATA