Stayfocusd for Chrome puts a limit on the time you’re allowed to spend on frivolous sites, and can be set to block EVERYTHING at certain times. It’s also very entertaining to set up. I found myself waffling around a lot during my “work hours” so I told it to nuke everything but Google Images during that time and it’s helped quite a bit.
I should make a web page full of links to all the productivity hacks I’ve found in my desperate search to tame my ADHD.