Every time I make a game, I end up reaching deeper into the inner workings of whichever engine I happen to be using. After years of using them, I took a shot at making an engine myself, based on features I've seen in the others. This is my third (I think), and most successful attempt and a reasonably easy to use and useful engine.
As it stands, I'd say it is as much a proof of concept as it is a fully-functioning library. Though it may never become a full game engine, I've been able to make some pretty decent prototypes with it with little difficulty. If you are interested, I encourage you to try it out. If it's missing a good feature you need, I'll add it in, or you can put it in yourself.