Hey! There's a new free and open source tweening, physics, and motion engine for Flash in AS3 that seems like it would be a big benefit for a lot of people here, called the Desuade Motion Package.

Here's some of it's features from the site:

MotionControllers: create pragmatic keyframe-sequences like CS4's Motion Editor
Sequences: sequence anything in the package in a very powerful way
Tweens: the heart of the package contains a tweening engine for standard tweening, bezier, and color tweens - that all use eases
Physics: similar to tweens, but have no end value or duration - uses velocity, acceleration, and friction instead of eases

Here's the site to learn more now: http://desuade.com/dmp

Or download the latest version directly here: http://desuade.com/download/dmp

Hope this helps out with anyone looking to code animations in ActionScript!