This is one of the coolest security feature that I have seen. WOT gets its data from users as well as other sources to show you if you are at adult site or spam site. It has plug-in for FireFox as well as Internet Explorer. I am going to try this one for few days.
Here is a screenshot of WOT when using it with http://update.microsoft.com It shows its rating on left. Since I have not specified any rating of mine, there are no ratings from me.
Things to improve
One thing that I find it lacking is that it does not validate if one person opens more than one account. Having a simple credit card validation (no need to charge the card, just use it to validate the user) for the users will give it a way to validate the users (They can also allow other users without validation with may be a little lower rating). If users will be concerned with their credit card details being shared..., than may be 3rd party authentication such as Paypal, Google checkout or Amazon Payment can be used. This is just to make sure that bad site owners don't spam the ratings.