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.
I should have reviewed this a lot sooner, I only just noticed I hadn't. Contra is really one of the top 3 mods in existence for zero hour. Featuring many new mechanics, a plethora of completely new and redone units... All the hours and hours of fun I've had with it, both alone and with friends accounts to so much pleasure that it really deserved a big fat 10 =)