limiting web surfing

I often spend more time web-surfing than I mean to. Telling myself I'll visit just one more site, then stop, doesn't work. I'd like to be able to set my browser or screen saver to automatically lock me out in a certain number of minutes for a certain amount of time.

While there would have to be an override option, for emergencies, it should be sufficiently unpleasant that I wouldn't override it lightly. For example, I might need to type in: "I promise I have a legitimate work reason for overriding the web stopper", and the computer would email someone or keep count of the number of overrides.