Limit By Domain by SamuelReed

Set limits for how many pages you can visit per domain. Doesn't block the homepage of that domain, but limits how many sub-pages can be visited per day. For example limit yourself to 5 YouTube videos per day, or 10 Reddit threads. (There is an exception built in that doesn't count the number of subreddits you visit.)
I wrote this purely for my own personal usage, but if anyone else finds it useful then great.