Event loop approach to concurrency

Event loop approach to concurrency as an alternative to threading - everything is non-blocking and executed via callbacks. The event loop is executing a queue of callbacks forever. This works well as long as the callbacks complete quickly! If a callback is going to take long it should fork another process. The primary application is networking - non-blocking I/O.
Douglas Crockford's presentation on event loop approach to concurrency

Libraries that use this approach include node.js, Ruby's Event Machine and Python's Twisted. and Java's new JDK7 Asynchronous IO and the older NIO library. The main difference between new Asychronous I/O and the older NIO - for NIO you are notified when the read operation is ready to start (data is available); while in Asychronous I/O - you are notified only when the read is completed (all data is read).

The design pattern being employed in all of these is the Reactor Design pattern. The Reactor pattern allows for the activation of handlers when events occur (e.g. activates handler to read data from socket when the data is available)