The LinkExtend addon for Firefox displays aggregate ratings for things like safety, adult content, and site traffic for web pages. Based on data pulled in from a wide variety of sources, these aggregate ratings can be very useful; just don't mistake this informative free/donationware tool for any kind of security software.
The donation-supported addon displays multiple ratings for the page you're viewing in a new toolbar. Clicking a particular rating summons a pulldown listing of all the different sources and individual ratings that went into the displayed overall rating. For example, clicking the Safety button will show ratings from Browser Defender, SiteAdvisor, Google Safe Browsing, and other safety feeds; info from Alexa, Quantcast and others go into the Site Traffic rating. The LinkExtend toolbar also offers a variety of site tools, such as gathered lists of all the links or files on a given page.
LinkExtend allows you to pick and choose which ratings to display and which services to use for which ratings. The various gauges will also by default display next to search results from Google, Yahoo, IxQuick and LinkExtend's own search service, but you can hide them if you wish. What's more, you can see all the ratings for a link before you visit it by using a right-click context menu option.
LinkExtend's huge array of gathered data can help you decide whether a site is safe, okay for kids, or generally popular. However, although it can block known risky or kid-inappropriate results from displaying in searches, it won't actively block those pages from displaying. Nor will it actively prevent malware downloads, so it's not meant as a first-line protection tool or parental control utility.
Also, in my testing there were a few times at which the toolbar icons displayed a green, low-risk aggregate rating for a certain screensaver site, despite its having multiple high risk ratings from individual feeds. A subsequent visit to the same site would then display a correct red, high-risk icon. And I also saw some inconsistencies in the search result icons. For some searches the icons would stay gray indefinitely; re-running the same search would usually fill them in.
Despite these seeming missteps, the sheer mass of site data made readily available by LinkExtend is a real plus for figuring out what's going on with any given site.