Glest is a project for making a free 3d real-time customizable strategy game. The game is available for several operating systems and can be completely modified using XML and a set of tools. The current version includes single player game against AI controlled players, providing two factions for the player to control: Magic and Tech, each with their corresponding tech trees, units and buildings.
In GlestNG you can get a 'modding' menu that lets you change everything about the game world. It will let you save your changes and package and distribute your changes too.