Socket.io - Realtime application framework (Node.JS server)
WebSocket-Node - A WebSocket Implementation for Node.JS ( Draft -08 through the final RFC 6455 )
Ably - Serious realtime infrastructure
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
eventlet - Eventlet is a concurrent networking library for Python that allows you to change how you run your...
PubNub - PubNub is a real-time messaging system for web and mobile apps that can handle API for all platforms and push messages to any device anywhere in the world in a fraction of a second without having to worry about proxies, firewalls or mobile drop-offs.