The Programmatic Tween Engine that I use and highly recommend is the TweenMax family from GreenSock. He has done an excellent job on continually improving an already robust engine, adding new features and new members to the family almost monthly.

The syntax is easy to use and very simple, he has also included the Penner Easing Equations into the package. I have been using it for about a year or so, and absolutely love it.