Functional Reactive Programming
There's something about FRP specifically that makes programming an absolute pleasure. Designing an elegant reactive application is a great feeling and opens you're mind to just how rewarding it can be to focus on decoupling. Thinking in reactive (message driven) streams has clear benefits between multiple applications, but even within a single app I find myself wanting access to a reactive library - because with FRP I think I have the most maintainable code, the most understandable abstractions, and have more easily kept my code clean and extensible.
Reactive resources and languages: