Warning: Before you read this, I’d better let you know that this is not a substitute for a good antivirus program.
Now, down to business, these two browser plugins will help you browse safer by giving you community ratings and/or by telling you what they found when they had bots scan the website. The first one is called McAfee Siteadvisor. (Which can be downloaded from: http://www.siteadvisor.com/) It has bots that go to the sites regularly and downloads everything that it can find on the sites to their servers and scans it. This is available for both Internet Explorer and Firefox.
The next one is called WOT, which is Firefox only (EDIT: There is a beta version out currently that is also compatable with Internet Explorer, it is somewhere on http://mywot.com), and can be downloaded from: https://addons.mozilla.org/en-US/firefox/addon/3456. It is based on community ratings.
You can discuss these two add-ons, and mention more (If you know of any more) in the forum