Site Advisor Browser Plugin
Avalible for both IE and Firefox (only tried the firefox version though).
It basically does some automated testing on websites. Checks downloads, annoying popups/popunders/exploits, completes signup forms and checks for spam then 'scores' the site. You basically get a little icon that's Red/Yellow/Green depending on the sites 'safety'.
You can go to the siteadvisor site and get more detailed information about paticular sites too if you wish. Other users can also comment on the results pages.