Opera Dragonfly is the Opera teams latest developer tool intended for people that need to debug Javascript, inspect CSS code and the DOM, and view Javascript errors. So it sounds like it's their own version of Firefox's Firebug, or the Web Developer Toolbar on Internet Explorer. Can't really say much about it other than it does most what Firebug does, but is for Opera and works with their Mobile browser too. If you use Opera, or develop for Opera, then this is going to be the perfect tool to help you. If you don't use or develop for Opera then I guess this won't be much use.