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:

• I don't need blazing fast bitcrunching performance. In this case I would strongly prefer ease of development over performance.
• I'd like to use a language with a healthy community, which to me means a fair amount of maintained 3rd party, libraries.
• I'd like the language to be cross-platform friendly, I work on a variety of different operating systems and would like something that is portable with minimum effort.
• I can't imagine myself using a language with out decent options for debugging and editor syntax highlighting support.

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