Web of Trust is a plug-in for your browser that lets you rate all websites based on their trustworthiness, reliability, privacy and child safety.

The software, which you download from Web of Trust, and supports Internet Explorer and Firefox, sends your ratings back to the company’s database so they can be viewed by others.

So far 16 million websites have been ranked, the company says.

After downloading, whenever you use a major search engine, you’ll see an icon next to each of the search results colored red, yellow, or green to indicate the site’s reputation. When browsing sites you can rate them by clicking on the plug-in icon in the toolbar and selecting from the color-coded bars for each of the four categories. This only takes a few seconds but leaves me wondering what fraction of internet users would bother going through the process for every site they visit. CEO Esa Suurio says that, as in all communities, the minority does the majority of the work.

The system uses statistical algorithms to calculate cumulative ratings as well as user reputation so as not to skew the results. Privacy is paramount for the company–no IP addresses or browsing history is ever saved, only ratings.

The Helsinki, Finland-based company raised $500,000 this past summer. According to Suurio, it will eventually draw in revenue by offering detailed ratings information to site owners.

WOT was released in September 2006 for Firefox and quickly grew to 100,000 users. Earlier this month a new version for IE and Firefox was released.