To get code from google code you may have to use a subversion client. On PC use the free TortoiseSVN.
- Glaze - super-fast, easy to use rigid body physics engine based on Chipmunk. Prettier syntax than box2D. Demo.
- Box2D - uglier (sorry) syntax than Glaze, but has more features (e.g. types of joint), and is better documented.
- PushButtonEngine - framework to build games on - doesn't do too much out of the box, but is an admirable attempt to standardise Flash game code structure. Demo.
- Collision Detection Kit - Pixel perfect collisions in Flash!
- Game Poetry (blog) and CheezeWorld (blog)- these excellent blogs post the source for many of their examples and tutorials on google code.
- PixelBlitz - engine for retro -stylee games. Blurb from Photon Storm.
- Flixel - another retro-engine. Cool Demo!
- PaperVision3D -if you want to go into the third dimension, this is the daddy. Also check out Away3D - it has some features that haven't yet made it to PaperVision3D. Hey, different strokes for different folks!
- Jiglib Flash - Physics in 3D!!!
- TweenMax - since fusekit was was sadly never ported to AS3, this tweening powerhouse has taken over the whole show. Not only is it great for animating your user-interface, menus etc, I have actually built whole minigames using only this library for all movement and animation. Also gives you quick ways to adjust brightness/contrast etc of images. Only downside is that it is time-based only so won't play nicely with your game if it has a frame-based tick - but this option has been added to the beta, and as you read this is probably now in the live version. If you need frame-based tweening, you can also check out GTween, although Grant is discontinuing the project.
What have I missed? Leave comments!