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.