I try not to do web development anymore, but it’s hard to avoid. I just encountered some CSS weirdness with (brace yourself) IE7, and wanted to look at what the CSS actually said, so I clicked “Show Web Inspector” on Safari’s “Debug” menu. I was completely blown away by the result. The tool has a very intuitive DOM viewer, which shows everything you could want to know about a node, and also hilights the result beautifully in the original page. The “Network” icon shows a great view of the load time for the page, helping to diagnose all of those slow loads. And the “Console” icon gives warnings from the XML/HTML parser! All those little typos that go unnoticed on all but one version of one obscure browser? Safari will actually tell you about them!
This is how you build a developer tool. Wow.