Programmers are increasingly taking advantage of tools built right into the browser, with the Firefox plug-in Firebug leading the way.
The Firebug ecology is so fertile that it has spawned a subcategory of plug-ins that extend Firebug itself, often in surprising ways. FirePython, for example, doesn't actually live on the browser; it gets inserted into the server, where it delivers debugging information to the browser.
Thanks in some measure to Firebug's popularity among developers, all the major browsers offer detailed information about the images, scraps of code, and whatnot that make up the page on view -- an approach that will only become more common as more software is written to take advantage of increasingly robust browsers.
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