Now WebKit includes a JavaScript debugger named Drosera. To use it you need to get the newest build of Safari and enter this in the Terminal:
defaults write com.apple.Safari WebKitScriptDebuggerEnabled -bool true
The best quote is this one:
One of the unique things about Drosera, like the Web Inspector, is that over 90% of it is written in HTML and JavaScript. This is a true testament of what you can do with web technologies today and the rapid development that WebKit allows.
This is cool.