If you are a fan of real-time strategy games Warzone 2100 will impress. This game works on Linux and will entertain for hours. The graphics are well done and the game play is also good. In 2004 the source code and most of its data was released under the GNU General Public License.
The game story is about a civilization that was wiped out after nuclear attacks. The survivors try to rebuild with pre-war technology. It is a 3D game. Here is the Warzone 2100 Resurrection site.
If you are using Ubuntu Linux use the Add/Remove applications to install Warzone 2100. Enjoy!