Contra is a freeware modification for Command and Conquer Generals: Zero Hour real-time strategy game. It's a big project started in 2004. It adds many new units as well as numerous new upgrades, new general's powers and buildings. It also adds new sounds, maps, bug fixes, enhanced graphics, and other effects, as well as three new generals. There are lots of people playing in multiplayer.
Amazing mod that adds many features, with few to none bugs. The mod is done by a team with professional level of excellence, and discerning attention to balance and detail.
Many units are added, and the game system receives a large improvement, yet the gameplay is still balanced, something not frequently encountered in other modifications.