ForerunnerDB - ForerunnerDB is the only JavaScript database with a simple, rich JSON-based query language.
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.
Alasql - Alasql.js - is a lightweight client-side in-memory SQL database designed to work in browser and...
Ably - Serious realtime infrastructure
PouchDB - Open-source JavaScript database inspired by Apache CouchDB that's designed to run well within the browser
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.