Stuxnet

This windows specific worm was discovered in July 2010 for the first time by an almost unknown security firm - VBA. This worm is first of its kind to target industrial systems. This worm is supposed to have caused the failure of a nuclear power plant and delay in start of another one in Iran. It is supposed that this worm was mainly written for that purpose as 60% of the infected machines are in Iran. Rumors are that this worm is also responsible for INSAT 4B failure and findings say that it has a 'kill date' of June 24, 2012 after which it will stop executing!

Although, harmless to normal PCs, this worm targets Siemens PLC which are used in several industrial establishments. This worm can reprogram the PLC's while the machine is working and go unnoticed. This worm is about half a megabyte in size which is quite huge for a worm and uses excellent techniques to break into a system. It uses four zero day attacks to attack the target. Its main spreading mediums are USB and P2P networks and it special in a case that it had digitally signed drivers of Realtek when it was detected and when the signature was revoked, it was found to have another signed drivers of JMicron. Possession of two digitally signed drivers made it easier for stuxnet to remain hidden for longtime.

This worm is very sophisticated and it is assumed that it may have took many man-months for a team of experts to make this worm. Even till now, its full effect haven't been found out and it's speculated that this worm could have been made only with a support of a nation and the fingers are up against USA and Israel. There are many more speculations about this worm and everyone's hoping to get more clarity on this type of worm as Kaspersky Lab stated it as "a working and fearsome prototype of a cyber-weapon that will lead to the creation of a new arms race in the world."

http://en.wikipedia.org/wiki/Stuxnet
http://www.symantec.com/connect/blogs/w32stuxnet-dossier