I've been trying out TweenLite (http://blog.greensock.com/tweenliteas3/), but it relies on the fl.motion.easing package, which is only available to paying Adobe customers.
However the actual code is by Robert Penner under a BSD-license: http://www.robertpenner.com/easing/
Based on the suggestion here (http://www.actionscripts.org/forums/showthread.php3?t=144172) ) I've taken the AS2 code and made an AS3 version, under the same BSD license. I cannot believe that no-one else has done this before, but if they have I certainly couldn't track it down.
TODO: put the zip file up here.