Ration your web usage.Blocks all websites unless you are running the timer.
I wrote this for myself to ration my web surfing to 25 minutes per day (though the extension doesn't prevent you from setting the timer multiple times in a day... yet).
This was adapted from Strict Workflow by matchu.
This is very beta, with lots of bugs. The source is at:
https://github.com/marcpickett1/webrationtimer