A little top-down defense game was released under GPL (art & code) as a new year's present. It shows nicely, how libre art can be used for small game programming projects.

These use some art assets from megamek (http://megamek.info/) and therefore these games are also licensed under the GPL 

A straightforward interpretation of the GPL applied to assets. I'm afraid though, the images of MegaMek might be incompatible with the GPL, due to Battletech copyright and/or trademark.

The game has good performance but little complexity except for a Missile Commander-esque twist: getting hit reduces player's spacial ability to shoot.

According to the project page, more games with the same graphical theme are to be expected. I'm looking forward to them!