Freeciv is an open source project that aims to provide a free Civilization clone (released under the GPL).
Quote:
WHAT´S CHANGED SINCE 2.1.0
* Built-in editor
* Expanded ruleset format:
* Unit classes
* Movement restrictions
* Can limit which terrains a unit can enter
* River-boats
* Wheeled units
* Which units another unit can transport
* Migration
* Plague
* Bases
* Calendar effects
* Additions to default ruleset:
* New terrains: Deep Ocean and Lake
* Triremes are not restricted to tiles next to land, but to (shallow) Ocean tiles
* Buoys
* Ruins
* New nations
* Some adjustments to how different power plant improvements work together
* Scripting improvements
* AI difficulty level: Cheating
* Rewritten borders system