Ration your web usage.
Blocks all websites unless you are running the timer. You get 3 rations every day, each 25 minutes. After you've used up all your rations, you can only surf in 1-minute intervals.
I wrote this for myself to ration my web surfing to an hour and a half per day.
This is now slightly less betas. The source is at:
This was adapted from Strict Workflow by matchu.