Over the course of a workday I tend to accumulate lots of browser windows, and even more tabs inside them. Up to now, I would often lose track of what which tabs were open in which window and in which space. In the end, I would often just open a page again in a new tab of the window I happened to be in at the moment, increasing the overall clutter.
With the advent of persistent application state across reboots or application restarts as well as fullscreen apps in Mac OS X Lion that situation has gotten even worse.
The "Window" menu in Safari does not help too much, because it only shows the tabs of the currently focussed window. Today, while wondering why a website was not displaying correctly, I accidentally found a remarkably simple (and built-in!) way of showing all open tabs across all open Safari windows.
Just hit Cmd-Alt-A or pick "Activity" from the Window menu in any Safari window to open or focus the Activity overlay window.
It is usually used to determine what servers different parts of any given website are loaded from. However, even if you are not interested in that, this little popup contains a list of all currently open tabs, regardless of the window they are in.
Double clicking any entry in there will take you to the correct window and focus the desired tab. Nice and easy, not even an extension needs to be installed :) The only drawback I see is that you cannot sort that list, clicking the "Address" column header has no effect.
A final tip: When you open the window, it might have one or more of the entries expanded, making the list overly long. Even though there is no collapse-all feature I could find, you can rather quickly do so manually by selecting the top entry in the list and then repeatedly hit the left and down arrow keys. Left arrow will close the currently selected item, down arrow move to the next entry. Even with lots of tabs listed, within a few seconds you should make your way all the way down, leaving a nice clean list of tabs.