Ctrl+t: new tab
Ctrl+w: close tab
Ctrl+q: close window
1: next tab
2: previous tab
Ctrl+g: disable stylesheets (toggle)
Ctrl+i: disable images (toggle between on, off and cached-only)
Spatial navigation is the one thing which keeps me on Opera over everything else. I mean, I liked the browser when I was still a mouse-user, it is lightweight and fast and standards-compliant (which is more use nowadays as more of the web is standards-compliant!). In Opera, to navigate spatially, you hold down shift and press the arrow keys, and it moves the focus between form elements and hyperlinks in the direction you wanted to go. I surf entirely without a mouse, and I'm a web developer by trade - it's pretty powerful.
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Are you an Opera User? What's your favourite feature?