Freeciv is a multiplayer and open source turn based strategy game. It is licensed under GNU General public license and it is free to download and use. It works on Windows, Mac OS and GNU/Linux. You start of as a tribe leader in the past and you have to guide your people through different centuries. It is important to discover new technologies so you can advance over time. You can wage war on your opponents or create diplomatic relations with them. The key is to get your civilization to be the largest, wealthies and the most advanced. If you enjoy turn based strategy games than you should give Freeciv a try. Check out their site below.