It's a small experiment with trying to get list of user's open tabs. It requires the pages to have a small tracker script, so probably you are not able to use it in practice – unless you're a Facebook engineer working on "Like it!" widget.
That page has a tracker built-in. To see it in action, just open the console and open this page a few times in new tabs – the result should be updated in real time. It is of course cross-domain, just check a tracked page on bzdety.comandeer.pl.
If you're curious how I did it – the tracker uses two techniques: cross-tab communication via storage event and communication between iframe and parent's window via postMessage API.
Tested in latest Chrome and Firefox.