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.
There is even a plug-in for Firebug called 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 🙂