Available in several weights depending on your needs, the Tween Framework provides for dead-simple programmatic animations.  The code couldn't be simpler:

TweenNano.to(mc, 1.5, {alpha:0.5, x:120});

as3corelib is a library of additional classes from Adobe.  It includes support for Json, cryptography, jpeg/png encoding, and a ton of array, date, and string utility methods.