The free mission, titled "Black Sheep," was originally intended to be the third mission in the game's Global Liberation Army campaign, but proved to be too edgy for EA to include in the final release. Says EA of the level:

...Players were given command of the Toxin Tractor, a slow-going farm vehicle modified to spray a deadly corrosive agent, and ordered to eliminate a town that had been "corrupted beyond salvation" by the USA's propaganda. While the mission was ultimately removed from Generals, the Toxin Tractor was available in other campaign missions, as well as in multiplayer and skirmish modes.

Head to the official Command & Conquer site to cleanse the corruption.