Occasionally ( read: when my fiancé allows ) I like to prototype different game or game-like ideas I have. Usually I use Java or C# (not xna yet) because they are the languages I have the most practice with. However I would like to learn something more suited to agile development; a language in which it would be easier to knock out quick prototypes.
At my job I have recently been working with looser (weak/dynamically typed) languages, specifically python and groovy, and I think something similar would fit what I am looking for.
So, my question is: What languages (and framework/engine) would be good for rapidly developing prototypes of 2d game concepts?
A few notes:
Note: If you are aware of a Java or C# library/framework that you think streamlines producing game prototypes I open to learning something new for those languages too
I used pyglet to prototype an iPhone game that I just recently finished. It was a joy to work with, and definitely rapid.