Kibosh от CK

This add-on is meant to be dead simple to use. The UI is bare bones, but should be intuitive after a short while.

From top to bottom there is
* The current timer (this is global)
* The max time limit (this is global)
* A list of sites which will be blacklisted when the time limit has been reached
* A text box to add sites to the blacklist
* An add button and a remove button

When the time limit has been reached, the remove button and timer will disappear from the UI until the next day.