Stuxnet is believed to be the creation of the intelligence agencies in the US and Israel. It is an advanced toolset specifically tailored to target Siemens industrial control system equipment. More specifically, it reprogrammed the PLCs (programmable logic controller) for centrifuges in nuclear enrichment at various facilities in Iran.
Built for sabotage purposes, the malware was stealthy in its actions and made it look like the damage it caused to the centrifuges was, in fact, the result of an accidental malfunction of the equipment.