Best 2D Game engine

Looking to make a DM-esque platform game.  I was just wondering if there's a freeware indie game engine that you don't have to pay for.  I've already looked at spritecraft, and I tried to use it for a while until I realized it was bullshit in it's complexity; looked at PTK, and was amazed by it's simplicity in comparison.  However, you have to pay for it, like, 200 dollars.  This is a hobbyist game, I don't want to sell it or have to pay to make it. 

So, any response?

I found Allegro to be a simple and fast means to put together 2D games. What does SDL offer that Allegro does not?

This is a serious question. I'm interested in hearing from people who have used both to make something non-trivial. Bonus points if you link to the projects you developed.