Google has a site to report sites harboring malicious code. I created a simple bookmarklet that will let you easily report any site you are visiting. It opens a new window and passes the url to Google's site, which then gets filled in on the form.

http://www.jasonblogs.com/tech/security/report-malicious-sites-to-google-with-this-bookmarklet.html

Nice idea. I wonder though if there are many people that visit a malicious page and also KNOW it's malicious?