"Opera Dragonfly is Opera’s debugging toolkit. It consists of tools such as a DOM, CSS and network inspector, JavaScript debugger, error console and command line. Opera Dragonfly is distributed with the Opera desktop browser"...
On the flip-side you could build your own debugger for Opera which used the Scope protocol to access what was happening in the browser.
I think this is something that the industry really needs, to decouple browser debugging, from browser plugins. I'm glad Opera are making this step.
BTW, Opera 10.5 is really fast. The Beta RC is still crashing quite a bit, but I really like the new features that have been added.