Prototype JavaScript framework

Prototype is “a JavaScript framework that aims to ease development of dynamic web applications,” sporting an “easy-to-use toolkit for class-driven development and the nicest Ajax library around.”
Ruby on Rails features integrated Prototype support, the famous script.aculo.us library is built on Prototype (but I curse Joshua Schachter for ever starting that ridi.culo.us URL trend), Rico offers Ajax components and effects built on Prototype, and so on.