The alt text to today's comic at xkcd.com (strip below) says:

After years of trying, I broke this habit in a day by decoupling the action and the neurological reward. I set up a simple 30-second delay I had to wait through, in which I couldn't do anything else, before any new page or chat client would load (and only allowed one to run at once). The urge to check all those sites magically vanished--and my 'productive' computer use was unaffected.

(bold is my emphasis)

Does anybody know of a browser extension or other software that will add this sort of delay? I've seen extensions which simply block sites, but not a delay like this.