Micropolis is an open source sim game based on the original code from SimCity. It was donated to the One Laptop per Child project by Electronic Arts under the GNU General Public License agreement. Micropolis is developed by Don Hopkins and has been available as open source since January 10th 2008.
If you’re interested in working with the code and creating your own SimCity game, visit the Micropolis Wiki Page. All the details you need in order to get started are listed on this Wiki page.
If you’d like additional help getting started, check out this great series by Bil Simser.
Don Hopkins is currently translating the source code to C++. This C++ version, known as Micropolis Core, is still a work in progress. The game itself is not yet playable, but you can scroll and zoom around the map. Visit the Micropolis project page on Google Code to see the latest news or to download the latest code for this version.