Stuxnet, as seemed obvious from the beginning, was written by a state power, two of them actually, the US and Israel. It succeeded in crippling centrifuges used by Iran in their nuclear facilities. It may also have made an attack on Iran by the US and Israel unnecessary.
Stuxnet targeted Siemens centrifuges in a specific plant in Iran only, making them spin fast enough to cause damage while sending innocuous data to those monitoring the systems making it appear everything was ok. This is not trivial programming, and made all the more complicated by it being launched elsewhere yet it was able to find its way to the target. Plus, it can receive updates.
It is curious that info about Stuxnet being developed by a joint Israeli / US team was deliberately leaked. Why?
The cyberwar arms race has begun. There will be many more Stuxnets. Maybe there already are. The most dangerous worms are the ones you don’t know are there.