FlatBuffers is an efficient cross platform serialization library for C++, C#, C, Go, Java, JavaScript, PHP, and Python. It was originally created at Google for game development and other performance-critical applications.
Access to serialized data without parsing/unpacking
Memory efficiency and speed
Cross platform code with no dependencies
But all the cool kids use JSON!
JSON is very readable (which is why we use it as our optional text format) and very convenient when used together with dynamically typed languages (such as JavaScript). When serializing data from statically typed languages, however, JSON not only has the obvious drawback of runtime inefficiency, but also forces you to write more code to access data (counterintuitively) due to its dynamic-typing serialization system. In this context, it is only a better choice for systems that have very little to no information ahead of time about what data needs to be stored.
Cocos2d-x, the #1 open source mobile game engine, uses it to serialize all their game data.
Facebook uses it for client-server communication in their Android app. They have a nice article explaining how it speeds up loading their posts.
Fun Propulsion Labs at Google uses it extensively in all their libraries and games.