Warzone 2100 is a fast-paced, Command and Conquer-type real-time strategy game. It was first released by Pumpkin Studios as a commercial product, way back in 1999, but it became an open-source project in 2004 and has been available for free.
The story is set in a post-apocalyptic late 21st century, where nuclear strikes have destroyed much of the world's civilisations and only scattered bands of scavengers remain. Your mission is to command the forces of "The Project", and rebuild the world with the aid of recovered technology. Sounds noble to us, but unfortunately three competing organisations have entirely different plans, and you'll need to win hectic battles on many fronts before order can be restored.
An extensive tech tree covering more than 400 different technologies means there are many different ways to play the game, and mastering it, and the many units available, could take quite some time. Fortunately a lengthy campaign and single-player skirmishes give you plenty of opportunity to learn how everything works, and you only need move on to the multi-player mode when you're feeling confident.