Aardvark FireFox Extension
Despite its odd name the Aardvark FireFox Extension is actually damn useful. Once installed, turn it on using Tools->Start Aardvark and move the mouse over the page.
As you hover over different parts of the page a red box will outline the current section, show you what HTML tag created it and show the elements “class” or “id” values. What’s less useful but still interesting is that once you’ve selected the element you’re interested in you can do a couple of occasionally useful things to it, remove it, colour the background, remove the element but leave a blank spot etc.