Fount is a free bookmarklet that can help you easily identify fonts on any website. To make it work, you need to add the bookmarklet and then activate it with just one click. After, clicking on any text will show you the font name, font size and weight in a growl-like notifications.
To identify other text on the page, you don’t have to re-activate the bookmarklet. And to disable it, clicking it is simply enough. This font identifier is a very useful and tiny friend for typography inspiration and it works in all major browsers.