Love this extension. Simple and elegant, allows you to block multiple websites with different settings, all free. I've tried others and this one has the most features. Nothing unnecessary - just super useful. Use it to curb procrastination during the work day and to resist online shopping, etc. - the different settings and block times for different categories makes this possible and stops you from constantly changing the settings to get around it! Thanks!

Update: still using it over a year later, still works great!