I've been working for about a year on a fun, free game development environment called Mini Micro, and it would be perfect for this jam. Check out the built-in platformer demo here!
If anybody wants to try this, you'll have my full support with learning the (very easy and elegant) MiniScript language, the Mini Micro API, and any general programming issues you might run into.
In fact I'm thinking about throwing in some sort of add-on prize for the best entry made in Mini Micro. @impeckable, what do you think of that idea? (It's just a thought; if you're not cool with it, I'll drop it like a hot potato.)