So just incase you are a big GTA fan or a fan of any open sandbox type games that go along with that, I would like to introduce the grit engine. 

 http://gritengine.com/

 We support large maps (e.g. more than 30 square km, 50000 objects) with considerable diversity in geometry and textures, because game objects and scenery consume negligible memory / CPU resources when out of visual range. We also use deferred shading, which is essential for rendering the abundance of light sources that are needed in open world scenes. The object of this is to allow people to create their own world with this engine and also to encourage the making of gamemodes and gameplay with Lua scripting. 

The source code and scripts of the Grit Game Engine project are open source and licensed for re-use under the MIT License

I will also be helping on this project trying to get a solid starting point experience with the game. Also with music and sounds free to use.  

 Videos of it in Action

http://www.youtube.com/watch?v=gARVzn3nBsQ

http://www.youtube.com/watch?v=Lh5LR0IssLM