ScheduleBlock

Allows you to block any set of websites on a schedule or based on a timer.
Allows you to block independently both opening new tabs and using old tabs (independently for each day of the week).
Allows you to block sites after amount of time is spent.
By default uses gray color scheme to save your eyes in the night.
Reliable, without spying, ads or microtransactions.
Fully open source, therefore open to both code inspections and great new ideas.
Perfect for improving your concentration when you need it.

If there are features you would like to see added, be sure to mention them in a review or on project's GitHub page.