Windows/Mac/Linux: Who knows better than you which tabs go together? That's the thinking behind Tab Stacking, a new feature in the latest Opera beta that lets you group together tabs based on site origin, theme or however you think they fit.
You can see Tab Stacking in action in the (smoothly narrated) video above. It's a pretty nice option to have when you're doing something resembling web research, and it's been put together smoothly. Hovering over a tab stack shows what's inside it, pressing the arrow expands your tabs back into the general population, and you've still got all the advanced arrangement tools (cascades, tiling and the like) to use on stacks of tabs.
This beta of Opera 11 also sees an opening up of the API for the new extensions, first seen in Chrome-like fashion in the Alpha. The usual speed, HTML5 and bug-fix improvements have been backed in as well.
Opera 11 beta is a free download for Windows, Mac and Linux systems.