Two 'towers' down in MiniMek Urbie Defense
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 GPLA 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!