Emissary is a Node.js library that provides mixins for subscribing to and emitting events, offering more functionality than Node's built-in event emitter. It includes Emitter and Subscriber mixins, allowing developers to extend objects with event-emitting capabilities or subscribe to events from other objects. This library is designed for Node.js developers working on projects like Atom, though it is no longer maintained and is being phased out in favor of the simpler event-kit library.