Originally intended to be the third mission in the Global Liberation Army campaign, the "controversial" Black Sheep mission was ultimately cut from the game and has never before been played outside of EA confines. In the mission, players are given control of the "Toxin Tractor," a slow-moving vehicle that sprays a deadly corrosive agent, and ordered to eliminate an entire town that has been "corrupted beyond salvation."
I'm guessing it was all that genocide stuff that led to the removal of the mission in the original release, but there's been a lot of water under the al-Sarafiya bridge over the past six years and I guess that kind of thing is okay now. Installation is simple: Download the mission, extract the file to the Generals Maps data folder, then run the game, enter Skirmish mode and select Unofficial maps, where the new mission should be listed.
Sure, it's only one mission and C&C Generals is a six-year-old game, but it's held up pretty well over the years, and more importantly, this is free stuff! Grab your copy of Black Sheep here.