I just published the ExpressionTimeline on Aescripts. It allows to limit the duration of expressions and comine them. You can for example modify a wiggle expression such that wiggles only for a certain amount of time. It works with arbitrary expressions and you don't need to touch the code of the expression at all. You can combine arbitrarily many expressions and even fade the expressions in and out smoothly over time. Finally, the most powerful feature is that you can create smooth transitions from one expression to the next one.
Sounds complicated? Its very easy to use, here is a short demo video:
The tool is available at aescripts.com for 29.99$ and until February 10, 2011, for special offer price 19.99$.