As another commenter pointed out, Opera already has something very similar to this. YMMV, but while I didn't really see the point at first, now I love it. I'm u

As another commenter pointed out, Opera already has something very similar to this. YMMV, but while I didn't really see the point at first, now I love it. I'm usually developing several websites at a time, each of which I have several tabs open for (one for the admin panel, one for the public side, misc ones for CSS tricks I'm experimenting with, etc). So I group tabs by what project they belong to, and when I switch to working something else I can just collapse the current stack and expand the other one. You get the best of both worlds: keeping your tab bar relatively clean and readable, and not having to put in all the URLs or search for relevant pages everytime you switch tasks (while sticking to a single browser window). Also the UI (at least Opera's) makes it very clear which tabs are actually tab stacks, and presents it in an intuitive looking way. People who have never heard of the concept and then use my browser typically grok it almost immediately just from the design.