Basically, I'm looking for a good, 2D game engine to build a platformer/contra-like shooter on to bolster (and when I say bolster, I really mean start) my portfolio.
I use the words "right now" in the title because I've read through some questions, and I don't really want to use anything like Java's game API for mobile phones from 2003. (Also, XNA is out of the question as I'm already working on a larger scale project in it.)
I'm fairly competent in Java, and have a basic understanding of other languages like C# and Python, but I'm totally unopposed to learning a new language while learning the engine (within reason).
The engine doesn't have to be super powerful, just free and simple enough that someone of my skill level (3rd year CS major) can get a nice horizontal slice out without too many bumps along the way.