Opera Dragonfly

Opera Dragonfly is Opera's all-new set of developer tools, designed to give developers a lightweight-but-powerful application that provides effective mechanisms for web standards debugging and problem solving without slowing down the browser, and fits in nicely with the development workflow. The current feature set is as follows:

1. JavaScript debugger
2. DOM inspector
3. CSS inspector
4. Command Line to allow commands to be inputed
5. Error Console that outputs validation errors and warnings exhibited by the CSS and JavaScript connected with the page
6. Proxy to allow debugging directly on mobile devices

The current release version is an alpha, so it is currently a little rough round the edges; also we decided to implement all the most critical features first, and go from there - we have big plans afoot for our developer tools and you can expect to see a lot of fantastic additions in coming months.

http://dev.opera.com/articles/view/introduction-to-opera-dragonfly/