For web developers on Mac OS X, there is a wonderful and powerful hidden tool in Safari that in part functions like the Firefox extension Web Developer. Though it does not have quite the same depth as Chris Pederick’s tool.
The Safari Debug menu can be activated with a simple command in OS X’s Terminal (/Applications/Utilities/Terminal.app). At the initial shell prompt typing ‘defaults write com.apple.Safari IncludeDebugMenu 1’ (with Safari closed) will add one additional menu item upon its next launch. By changing the 1 to a 0 the Debug menu can be turned back off.
There are more handy tidbits – for instance opening the same page in multiple other browsers installed on your Mac for preview tests via a single click within Safari, and some limited SSL debugging.
Finally – a nice little bookmark importer for Mozilla, IE and Netscape (sorely missing in Safari without the Debug menu on!) and the capability to export Safari bookmarks rounds it out on the convenience scale.