The "Inspect Element" tool built into my browser, Google Chrome, is one I use all the time. I get design ideas from other websites, test-drive tweaks on my own site, get information about images and dimensions, find out the names of divs and classes in CSS and all sorts of other handy information. It's a bit hard to explain, so I'll just show you how it works. Check it out:
Can't see the video? Watch it here.