emitter.io - Real-time communication service for connecting online devices.
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.
SignalR - SignalR is a server-side software system designed for writing scalable Internet applications, notably web servers.
Ably - Serious realtime infrastructure
Entity Signal - Real Time Entity Framework (EF) and ASP.NET core. Make all of your data real time.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.