Flixel

Flixel is a free ActionScript 3.0 (Flash) library for 2D, raster-graphics game development. I came across it recently while looking for tips to speed up my own Flash raster system. Flixel is by Adam "AdamAtomic" Saltsman, and is released under the MIT license (yes, that means you can use it in any project for free).

Flixel uses hardware blitting, bypassing Flash's vector renderer, to render raster graphics very quickly. It also provides a framework for animating graphics, game states, collision detection, text, buttons, tilemaps, and more.

If you have a basic understanding of ActionScript 3.0, you can get started pretty quickly. If you need help setting up Flixel in FlashDevelop, there is a tutorial on that here. I found the documentation a bit lacking in some spots. Perhaps to make up for this, there is a full game source included with Flixel for you to dissect and learn from.

Overall, the library is incredibly easy to use. However, the lack of any real "Getting Started" tutorial, to walk you through making a basic game and introduce you to the various systems in place within the engine, may make the engine too difficult to learn for beginner programmers. Hopefully, as the engine develops further, more tutorials will pop up to remedy this.