Opera has many interesting features. One of the most useful feature for webmasters (and not just for them) is "reload from cache". Load a web page, view the source and edit the code. After you finished, click on "reload from cache" and see the updated web page.This way, you don't have the edit code locally, preview in the browser, upload it and then preview the page again. It's also useful if you want to create screenshots for a page that contains sensitive information. For example, you can remove the username from Gmail and other Google pages.
Opera also has a toolbar for web developers, similar to a well-known Firefox extension.
Update: For Firefox, there is a much more complex extension that lets you modify a Web page from the browser. It's called Platypus.
Related:
10 great features from Opera
