In Flash there are a multitude of tweening libraries available to developers from the built in tween engine, to libraries written by developers for both AS2 and AS3. Currently the TweenLite and TweenMax libraries (by Jack Doyle and avaiable on his website GreenSock) are the ones I currently use in my Flash projects for a multitude of reasons the main being that TweenLite has low overhead in file size, easy to implement, and has excellent performance.
Currently v11 is underdevelopment with the beta release available to Developers. The most promising new feature of this release are the TimelineLite and TimelineMax classes. These classes allow for users to easily create and manage sequences of Tweens. Enhancements have also been made to the TweenLite/TweenMax classes to allow the user to use frames instead of seconds for their tweening parameters allowing smoother animation in some scenarios.
To read the full details of the upcoming release visit the v11 Beta page on the official website.