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.
The content is creative and well-designed, but it definitely takes away much of the semblance to the original Zero Hour.
What bothers me the most is that this mod took away something so fundamental as selling most buildings. It really removes several tactical options (like selling a building before it is captured) and forces the player to make permanent decisions about their base layout early in the game. While more realistic, I don't feel that realism is a dominant theme of this mod. That's not that realism is good or bad; the mod just seems to go towards both ends of the spectrum to the point of incompatibility.
Also, the AI has some significant bugs, and oftentimes crashes will end an otherwise enthralling round (which, gameplay-wise, ultimately reduces to either an early rush or a tech rush depending on the length of the game).