Tetragon is an Open Source cross-platform game engine used to develop games and applications for the web, desktop and mobile devices. It is written purely in ActionScript 3 and comfortably allows to write source code once and deploy builds to several different target platforms. Unlike most other Flash-based game engines and frameworks that allow you to develop a specific type of game or that provide a subset API to support game development, Tetragon provides a complete foundation with that any type of game can be developed, incl. resource management, debugging facilities, multi-locale support, layered extendability, a game-oriented data structure, and a lot more.
Here are some starting points: