Its most valuable feature is the ability to view the generated source of a page—that is, to see the HTML as it exists at the current moment, including any changes that have been made via JavaScript. It's absolutely indispensible for any kind of AJAX-ish work.

It can do a lot more than that, though—with a couple of clicks, you can validate documents, see what your page looks like at different resolutions, quickly view or delete cookies from only the current domain or path, view overlays which reveal various aspects of the document (such as the names of form fields or information about links), disable images or CSS, autopopulate forms to accelerate testing, and much more.

JSView adds a submenu to the context menu which lists all stylesheets and JavaScript files linked to a page, along with the file sizes. Selecting a file from the menu will display the source in a new window. What JavaScript library are they using on that page? How did they get that effect with CSS? JSView makes it easier than ever to find out.

It's helped me to diagnose obscure bugs, to understand the workings of third-party web applications (such as when working with Microsoft products, which tend to freak out if the headers aren't set exactly as expected), to discover the URLs of embedded media files so I can download them (even on sites which obfuscate the URLs through various means), and more.