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.
The Contra 008 Final tournament has already begun. You can see the progress here: Cyberbattle.org
You can also download replays from finished games. Click on a match and you will be brought to the match page. Click on the "Download" button there.