A feature that tells you how many hours you have spent on a site.
I spend most of my time on the same sites (reddit, youtube etc.), and I would find very informative if I could see how many hours I've spend on certain sites, that way I can see if I spend more hours watching youtube videos, than studding, or doing something productive.
It would also be good for me to know how many hours I've spend on these sites today, and maybe stop browsing the web for an hour or two.