Contra is a freeware modification for Command and Conquer Generals: Zero Hour real-time strategy game and one of the first mods for this game. 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.
Apart from the AI pathfinding issues causing disgusting framedrops, this mod is very fun even in it's current state. Luckily, it seems like the developers are working to improve the game in a unique and interesting way, especially with how they integrated the ranking and the tech-tiers.