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.
A pretty fun and ambitious mod which adds a lot of unique new content and a very distinct look and feel from the original ZH. Overall the mod focuses even less on realism and more on adding as many fantastic units and weapons you can imagine, but without going totally overboard.
Overall this is a pretty deceny mod, and the latest 008 release is a huge improvement over 007. Many of the unit designs are creative and unlike anything you'll see in a typical Zero Hour mod, ranging from Cyborg infantry to giant robots to tanks the size of battleships, "real" nukes, and giant lasers which can intercept Nuclear missiles. While it may sound outlandish, it's fairly well-polished and never feels completely over-the-top. It also introduces some unique gameplay mechanics, such as tech buildings which rebuild themselves when destroyed, larger supply stashes, and requiring the players to rank up before gaining access to their most powerful units in their arsenal.
That's not to say it's perfect though. My #1 major problem with this mod is that the supply stashes have been made to have virtually infinite resources (almost $1,000,000 per stash versus the original $30,000). This essentially makes all secondary sources of income useless (hackers, oil derricks, supply drop zones, etc) - just build 10 Chinooks or Supply trucks, or 20+ GLA workers and you'll never have to worry about running out of money. This makes the game feel a little dumbed down, and places emphasis more on just pure unit spam than careful resource management.
Aside from that the game is pretty unpolished in some areas: Some unit voices and camos don't fit. Some unit models are noticeably better than others (though all models are now original and you won't find any of the 'mish mash' models from 007), some units are ripped from other franchises (Terminator, Cylon, etc) the Challenge mode is currently bugged and unfinished, and there are some noticibly overpowered units, as well as too many redundant units (ex. does Air Force General really need 13 different planes? Does Super Weapon General really need 6 different types of base defenses? etc)
Overall though while this mod is not for everyone, it has good potential and is one of the most ambitious mods for Zero Hour.