When you're working on a project, time is almost always working against you. That is why I rely on classes for my projects. Classes in its most basic definition are a set of reusable functions or methods. The keyword here being reusable of course. A good class can be applied and reapplied and because of this, is a huge time saver.
However, I'm not going to talk to you about Object-Oriented Programming or how to create your own custom classes in Actionscript. I -am- going to talk to you about classes that are already out there and that you can use freely.
There are two classes that you will almost always need when working on a big or advanced project; a transition class and an XML parser class. (With the introduction of AS3 and E4X, the need for an XML parser has somewhat declined).
Tweener