Opera has released Dragonfly, their Firebug like developer console, as part of the latest release Opera 9.5 beta 2. It looks pretty promising - at a quick glance I think it compares favourably with the equivalent tool in Safari 3.
The initial version includes a JavaScript Debugger, a DOM Inspector, CSS Inspector, error console and a command line and, although it doesn't currently offer editing capabilities, they'll follow shortly as well as the ability to inspect XHR/HTTP headers.
I used Opera as my main browser for a couple of months at the start of the year but found myself reluctantly returning to Firefox as I was having to run it almost all the time anyway for access to the developer tools. The addition of editing capabilities to Dragonfly will likely see me return to Opera as I found it much snappier and memory usage over time much lower.
More information and download details available from the official Dragonfly web site. Chris Mills has also posted a detailed walk through explaining how to use the tool on the Opera Developer site.
It's an interesting thing but I prefer FireBug.