Stuxnet is one of the most famous computer worms that was discovered several years ago, in the middle of 2010. The first time when Stuxnet was noticed, it was targeting Iranian nuclear facility with a reason to injure country’s uranium enrichment program and prevent President Mahmoud Ahmadinejad from building a nuclear weapon. As you can see, this is not a typical worm - instead of trying to steal credit card details, passwords or other sensitive information, Stuxnet is launched against industrial systems. It basically cause the centrifuges to go nuts and self-destruct causing lots of damage. According to security experts, this threat has a great potential to be used for physical destruction.
It seems that attackers have designed this digital worm with a great care so that it wouldn't hit those computers and networks that do not meet specific configurations. According to some security experts, it is also set to apply some self protection tips, like easing itself on June 24 2012 and similar. The way how targets get infected with Stuxnet is really unprecedented - this virus has already used four zero-day vulnerabilities. In addition, this half a megabyte in size virus can also infect system through the removable drives, such as USB drive or similar.
In the end of last year, security experts noticed a new threat (duqu) that seems to be created from the same code base as Stuxnet. However, having almost identical code base, this virus seems to be released for completely different reason than its predecessor – it seems to be designed for information theft.
Other Viruses in the Stuxnet family
Video on how stuxnet infects PLCs 
Article about Stuxnet on PC MAg