IPvFoo is a Chrome extension that adds an icon to your location bar, indicating whether the current page was fetched using IPv4 or IPv6. When you click the icon, a pop-up appears, listing the IP address for each domain that served the page elements.

Everything is captured privately using the webRequest API (new in Chrome 17), without creating any additional network traffic.

Chrome Web Store Link:

https://chrome.google.com/webstore/detail/ecanpcehffngcegjmadlcijfolapggal

Screenshot:

Firefox Port:

Another developer, who goes by "Dagger", created a re-implementation of IPvFoo for Firefox, known as "IPvFox". It uses the same concept and images, but different code.