[callahad]

I saw a particularly impressive one the other day which watched for mouse events on document.body, and when your mouse left the page in the "up" direction, it triggered a "Wait! Don't switch tabs! Check out this offer we have for you!" popup.

[cyxxon]

Sounds nice. Just yesterday I saw another version of not too intrusive popup - it came after about 20 seconds at the bottom right, not obscuring the content, but noticable (some animation, don't remember exactly), and in the popup the headline was "Hey, at least it's not centered" or something like that. I actually smiled at that, so it got a positive initial impression. I didn't sign up because I wasn't that interested in the content of the site, but still nicely done.

[gk1]

Those are called "exit intent" popups. The idea is this: If the person is about to close the page anyway, why not take a shot at offering them something? There's nothing to lose.

[sukilot]

Except getting your site on a junkware blacklist.