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.
This has to be the best mod I have seen in a long time. Graphics are great and game play is smooth despite the few bugs, but that is to be expected. The computer is challenging enough to make a good fight, and the weapons are balanced in a chaotic sort of way, which was just what I needed to spice up my C&C Generals Zero Hour game play.