Stuxnet is a virus that was discovered in June 2010. While it spread to just about any computer without prejudice, it only caused issues with infected Siemens industrial equipment and software that was running on a Windows system.
From what security and research companies could determine, the worm appeared to be targeting uranium enrichment infrastructure. It did the most its damage in Iran, but also affected Indonesia and India. Due to the sophistication of Stuxnet, there is speculation that the US and Israel may have developed and distributed the worm with an intent to 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.
Tip: For those who want even more information about Stuxnet can also watch the documentary Zero Days.