Chris blogged on PhatBoyG about the Web Inspector included in Safari 3. Previously this tool was a simple content inspector, but now it looks to be so much more. If you are using Safari V3 and you are a web developer, have a look at this useful tool.

You can gain access to it by enabling the debug menu. Open Terminal and type this
defaults write com.apple.Safari IncludeDebugMenu 1
then restart Safari. You will then see the debug menu and the Web Inspector option. As Chris pointed out, the network tool looks fantastic for diagnosing page loading speed issues.