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.

Features:
Introduced Ranking system, in which certain buildings and units can only be built when a certain general's rank is achieved. Players get awarded for their combat efficiency.
3 New Generals - USA Cybernetic, China Flame and GLA Assault.
A.I. is greatly enhanced.
Introduction of "Super Units". The limit is 1 per player (like hero units). Super units require 5th rank. Each general has a unique super unit.
Neutral Tech Structures reappear after a while after they are destroyed.
Deep variety of gameplay tactics and strategies.
Wide choice of units, buildings, special powers and upgrades.
New graphics and sounds.
All 12 generals were made as diverse as possible. The gameplay for each general is unique.