Flixel is a free FlashActionscript library created by Adam Saltsman (aka Adam Atomic). It provides some base classes for creating 2D Flash games and other content. It was originally built up from Saltsman's work on several games such as Gravity Hook, Fathom, and Canabalt.
Features
Animations
Collisions
Movement
Tilemaps
Particle effects
Visual effects such as flickering, screen-shaking, blend-modes, alpha adjustments, etc.
Simple physics (Box2D can also be used with Flixel)
Keyboard and mouse access
Audio controls for volume and muting
Pausing gameplay
Site-locking
Pre-loading
See also
Stencyl, a game creation tool with Flixel as the underlying engine