Free Civ

FreeCiv is a FOSS (Free / Open Source Software) implementation of the Civilization family of turn-based strategy games. Several rule sets are provided allowing for different types of game behavior. In addition, because the source code to the game itself is available, a sufficiently motivated and knowledgeable person can implement changes to the game's behavior beyond anything possible merely by changing the rule set the game uses, including behavior not found in the commercial precursors to this game.

Most things involving it (including downloads) can be found at its wiki.

Tropes used in Free Civ include:

Artificial Stupidity: Computer players are extremely stubborn with "their" territory - build a city on any square they consider to be "theirs," and they'll raze the city - without any diplomacy scene or change in relationship. In fact, if you then attack one of their cities, they'll blame you for starting the war.
Fan Remake
Gameplay Automation: There are build lists that can be applied to new cities so that they automatically construct improvements and troops in the order desired by the player. Cities will also autobuild unless coinage is put in the build queue.
Technology Levels: As standard for Civilization games, it uses this only to a certain extent. For example, it is perfectly possible to build Ironclad units without having researched Iron Working, and to build and launch a spaceship without ever having discovered Sanitation.