Symantec’s Web site ratings service Norton Safe Web presents the Dirtiest Web Sites of Summer 2009 – the top 100 infected sites based on number of threats. Norton Safe Web is a new reputation service from Symantec. 

What makes these sites so dirty?
Symantec explained it by pointing out the fact that the average number of threats per malicious site rated by Norton Safe Web is 23. With that said, the average number of threats on the Dirtiest Web Sites list is a staggering 18,000 per site. Forty of the top 100 have more than 20,000 threats per site. Moreover, 75-percent of sites on the list have distributed Malware for more than six months.

“This list underscores what our research shows. There has been exponential growth in the number of online threats that are constantly evolving as cybercriminals look for new ways to target your money, identity, or assets. In 2008, most new infections occurred while people were surfing the Web,” said Rowan Trollope, senior vice president, Consumer Business Unit, Symantec.

Here's the link 
 http://safeweb.norton.com/dirtysites

This is really a well job done by Symantec,Hats off to Symantec ! Apart from the list what other awesome specialties I liked about the service are.. 

-You can check a site if it's safe or not
On the top right corner of the page you'll see "is this site safe ? enter site address box" you can paste any site link there and find if the site is safe or infected.

-You can write a review of a Web site
"Share your stories and experiences—good or bad—about any Web site that you feel strongly about!
Help others avoid the same pitfalls that you've gone through on some sites. Plus, let us know which sites are extraordinary and exceptional. "

-Quick in-depth reporting  

The reports for the websites are quick and in-depth with the exact malware/exploit name and link,which is awesome...

-Site Owner link 

It may be possible that a website is infected or exploited with out the site owner's knowledge.There's a link for the site owners http://safeweb.norton.com/help/site_owners where the site owners can reach Symantec and further communicate with the experts.

-Community Buzz

There's an active community where users can share their reviews,check latest threats [infected websites] etc.
http://safeweb.norton.com/buzz

This step taken by Symantec is really would help WWW to be a safer information super high way.