PC World has published a list of the 17 most dangerous places on the web, you can read the article over here. Here's one of them:
Threat 1 >> Malicious Flash files that can infect your PC
The Place: Websites that use Flash
Adobe's Flash graphics software has become a big malware target in recent years, forcing the company to push out frequent security patches. But another danger you might not know about is associated with Flash cookies. Flash cookies are small bits of data that their creators can use to save Flash-related settings, among other things. But like regular cookies, Flash cookies can track the sites you visit, too. Worse still, when you delete your browser's cookies, Flash cookies get left behind.