For web development, a laptop with a case fan that starts screaming like a jet engine when your cpu load skyrockets is sometimes a Really Good Thing, since it gives you cues to what happens in your browser, whether you set out do do profiling or not when, bad enough things happen. (A bit like how you could debug your programs on the C64 from the overtones that would leak from cpu to audio circuitry, giving distinct audio signatures to the occasional choke point in your code.)
Anyway, a Swedish social site I occasionally frequent used to have this effect for me, and get worse the more tabs I had open to it (and it was bad already with a modest three). I used to think it was a bugged Flash app (which they use for ads), but when I realized I had killed them all with AdBlock and FlashBlock, it occurred to me that it might be run-amok javascript.
Only that javascript that runs amok usually gives you the annoying modal dialog asking whether you want to keep running the cpu hogging script. That, however, only happens when page javascript hogs the cpu from the UI, so the single UI thread gets starved, and you, the user, get annoyed with an unresponsive UI. That can be avoided by running smaller loop fragments that call one another with setTimeout calls, leaving some breathing room for the UI thread in-between.
So how do you debug if that is what is happening? Well, for one thing, at least in Firefox 2, you can call setTimeout yourself two times in a row, some second or so apart, and compare the timeout id:s you get back, because consecutive calls get consecutive id:s. So if the difference is one, no setTimeout call was made in-between.
I crafted a bookmarklet to count setTimeout() calls per second (well, I honestly started with something smaller in the Firebug console, but figured it was useful enough to save, share and reuse later), and sure enough, that site was calling setTimeout 178 times per second. (It's not very interesting to run it on this site at present, but you can always race with yourself and the UI responsivity of your browser, attack clicking it to see how many setTimeout calls you can fire off in a second by hand. I believe it is not one of the performance tests we will see in any side by side browser shootouts any time soon. My personal highscore with Firefox 2 is 3. :-)
To do what? Well, time for another bookmarklet that shows which arguments setTimeout gets called with (it won't run on Internet Explorer; sorry: kindly advise Microsoft to implement the toSource prototype for all javascript native datatypes -- I would want to say it would not be a decade too late, but that would make me a liar :-), and lets you restore the original setTimeout implementation when you are done peeking.
It turned out the site was trying to fetch a node with an advertisement I had AdBlocked away. Over and over again. A tiny Greasemonkey script later, and the laptop fan was happily put to sleep, once again. A toast to audial debugging!