For those of us who do web-based development, Firebug must be the single most useful tool invented in a long time. I’ve used it for ages but I keep discovering new stuff it can do.
If by any chance you haven’t discovered it yet, go and get it now – it’s free. Any time spent learning your way around it is definitely going to be worthwhile. For some more advanced examples, you can watch Joe Hewitt’s talk, given last year at Yahoo.
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There is even a plug-in for Firebug called YSlow:
http://developer.yahoo.com/yslow/
It’s mainly for analysing performance, but also has useful tools built in like JSLint for checking code quality.
Oh dear, says not compatible with Firefox3.0 🙁
It should prompt you for an update, to the latest version which is 🙂