First off:
SuperTux - this is an extremely fun, extremely well-done Mario clone, featuring everyone's favorite penguin mascot. It even features a level editor. It is open-source, and already available on *nix, Windows and Mac OS.
![Image](http://super-tux.sourceforge.net/images/game-010_small.jpg)
![Image](http://super-tux.sourceforge.net/images/game3-010_small.jpg)
![Image](http://super-tux.sourceforge.net/images/menu-011_small.jpg)
![Image](http://super-tux.sourceforge.net/images/leveleditor-011_small.jpg)
Secondly:
TuxPaint - a very nice children's painting program that allows you to make your own pictures, or color in pre-made pictures. It also features pre-made stamps, so you can stamp different things into your pictures. I don't know if it would even be remotely possible to port this to the DC, but it would be very cool to see, if it was possible. It's written completely in C, and is available for *nix, Windows and Mac OS.
![Image](http://www.newbreedsoftware.com/tuxpaint/screenshots/starter-coloringbook-t.png)
![Image](http://www.newbreedsoftware.com/tuxpaint/screenshots/tool_stamps-t.png)