Hopefully I'll get a chance to cover each of these in more detail shortly, but there's a whole bunch of cool new Actionscript 3 projects that are floating around at the moment - here are some of my favourites that I'm trying to find enough time to explore in detail. Each stands out from the crowd for being pretty awesome, and I'm hoping to find some killer way to combine them into my projects.
For Animations & Tweening:
GreenSock Tweening Platform [link]
The name actually encompasses five different varieties of animation libraries, and is awesome at scripting individual and timeline animations. Not just that, but it comes packed with a load of animation options that you didn't know you wanted, until you see them. Like realistic motion blur. That one's cool. Especially once you've tried making it yourself in the past...
Collision Detection Kit [link]
Much like the GreenSock stuff, just how good this is really has to be seen to be believed. The basic Actionscript hit-test tools were never really that great. Sure they worked, but if you wanted anything more than a Boolean response, it was no good at all. The Collision Detection Kit gives you enough information to do proper physics - working out things like what, where and how hard things hit. It works with any display object - vectors, bitmaps, even video.
Box2D. Runs blindingly fast, precisely calculates physics and interactions. It's a true physics engine, rather than others (like Collision Detection Kit) which are just imitating physics. A little hard to get into because it's all written differently to every other Actionscript example I've ever seen ("You must unlearn what you have learnt") but once you get over that hurdle, it's just so... powerful.
Documentation for Box2D isn't very new user friendly, but Emanuele Feronato has a fantastic series of blog posts on Box2D which will prove invaluable.