I previously introduced ReactiveCocoa. If you are looking for a good introduction, Ray Wenderlich has a two part tutorial that may be of interest. ReactiveCocoa Part 1 and Part 2
For the uninitiated:
ReactiveCocoa is an Objective-C framework that provides APIs for composing and transforming streams of values. By chaining, combining, and reacting to signals, software can be written declaratively, without the need for code that continually observes and updates values.