One of the comments mention an API that could be used to integrate it with other browsers. I hope he was right, because it would be very cool to have just one debugging tool for all of the browsers out there.

Not bad, Opera is a nice browser and this thing is way overdue, but it's good.

Safari users can enter the following in Terminal (and restarting Safari) to enable the Web Inspector and other stuff available in Safari:

defaults write com.apple.Safari IncludeDebugMenu 1

It's a good tool. Definitely comparable to Firebug.

Or, in Safari 3, just go to the Advanced tab in Preferences and activate the "Show Develop menu in menu bar" checkbox.