tracking visitors after they have left my site

Just wondering if it is possible to track what the user does on other sites. Say for example a user comes to my site and I track his actions with a cookie. Now is there any way that I can track this user's activities after they have left my 开发者_开发问答site?

---

It's impossible since cookies are being tied to domain they are being set in. HTTP State Management RFC has more on this.

---

Impossible? Not that I think it's an ethical thing to do, but could some javascript event handler be established in hidden window to note when navigation events occur? Send info back to originating server? Might not be reliable but maybe some snooping is possible?

Oh, and there's this: http://static.whattheinternetknowsaboutyou.com/results.html