It's not technically a game, but it does use the Unreal Engine, and it is very cool.

When game developer Christopher Albeluhn found himself out of the job after working in the industry for seven years, he used his new forced free time to create an accurate model of our solar system using the Unreal Engine. It features accurate constellations, orbits, planet rotations, locations, and speeds.

Albeluhn started an IndieGoGo page to start taking donations on the project with the ultimate goal of releasing it on PC, along with other platforms like iOS devices. Albeluhn met his financial goal, and is now interested in donating the application as an educational tool to schools and museums. You can find out more about the project here.

One thing I can't help but think about, after just finishing Mass Effect 3, is how awesome it wold be to explore Albeluhn's accurate three-dimensional galaxy with the Normandy. The galaxy exploration sequences in all of the Mass Effects always felt lacking to me. All I am saying is, maybe BioWare should look into hiring Mr. Albeluhn to help with whatever is in store for the future of Mass Effect. I hear he is looking for work.