Do you find this useful?
newobj: I think it's a pretty much perfect implementation of a service of this nature. Esp. like that I can use bookmarklets. How robust is your frame buster buster? I do get a slightly weird interaction when loading nytimes, at least in chrome. A killer feature would be pausing if you tab away, but I'm pretty sure there's no way to do this. Having said that, I don't really see myself using something to limit my time on a page.