Stuxnet
Stuxnet is a virus that was discovered in June 2010. While it spread to just about any computer without prejudice, it only caused issues and actually infected Siemens industrial equipment and software that was running on a Windows system.
From what security and research companies could determine, the worm looked to be targeting uranium enrichment infrastructure and did the most damage in Iran, but also affected Indonesia and India. Due to the sophistication of the worm, there is speculation that the United States and Israel may have had a hand in developing and distributing the worm, with intent to slow-down or cripple Iran's nuclear program. However, no proof has been found to determine the truth behind the speculation. Below is a great video overview of this virus.
Also see: Security definitions