Flixel is a popular open-source game development framework designed for creating 2D games using the ActionScript programming language. It simplifies the game development process by providing a set of tools and libraries that help developers manage graphics, sounds, and user input efficiently. Originally built for the Adobe Flash platform, Flixel allows developers to focus on gameplay and design rather than low-level programming details. Its user-friendly structure and extensive documentation make it accessible for both beginners and experienced developers looking to create engaging games quickly.