I love Opera to pieces, but ever so occasionally, for one reason or another, a site just doesn't agree with the browser. So a long time ago I found this nifty little Opera web development add-on called W3-Dev:

http://tobyinkster.co.uk/opera

Follow the instructions on the appropriate install and choose what options you want to show up in the toolbar (the 2.91 should still work if you're on Opera 9 beta 2). If you wish, you can also add options for Firebird, Konqueror, Lynx, and Dillo (no Epiphany though).

Now my whole reason for downloading it was the View in Firefox and View in Internet Explorer options (back when I was on Windows XP), but for you web developers or aspiring web developers, there's also a whole bunch of stuff like links to web standards, validators, help with various scripting stuff, and some other neat stuff. Can find another web development add-on here:

http://operawiki.info/WebDevToolbar