Most useful ActionScript packages / libraries

Question

What are some useful plug-ins, packages or source-code for ActionScript?

Please include ActionScript version, name, link and description.

Answer 1

Flixel

Flixel is an open source game-making library that is completely free for personal or commercial use. Written entirely in Actionscript 3, and designed to be used with free development tools, Flixel is easy to learn, extend and customize.