APIs are becoming ubiquitous, but they are really hard to design well. In this talk, we'll discuss how to design and implement an API that isn't just functional, but makes people stand up and cheer. We'll also cover tips for integrating with other people's APIs. But an awesome API isn't just a feature. APIs are currently transforming the world, just like open source software has changed the world for the last decade. We'll talk about how this transformation impacts developers and changes the rules.