The past week I made a level editor for top down style games... it's very simple, stores no metadata... I want to share it here along with my GUI that I used to make it. It's all done with Allegro 5. Source code included.

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For loading the level files into your game:

All data is stored little endian.

First three 32 bit ints are level width, level height, number of layers.

After that, #layers WxH blocks as follows:
32 bit tile number
1 byte sheet number
1 byte solid/passable

And that's all. It's all BSD licensed. Build system is just batch files right now... binaries are also included for Windows.

Nice! If you don't mind, I think I'll just steal your A5 GUI. How much stuff does it support?

EDIT: Oh, it looks like it only has some basic widgets. I guess I'd need to make most on my own.

Yes, the gui itself has no widgets. You can steal the widgets from the editor or make your own. The splitter widget in the editor is very handy and it's how I do all of my layout. It's also the most complicated widget, so feel free to snag it.