OS X productivity application to block access to different websites?
I use Antisocial for this:


  Anti-Social is a productivity application for Macs that turns off the social parts of the internet. When Anti-Social is running, you’re locked away from distracting social media sites, including Facebook, Twitter and other sites you specify. With Anti-Social, you’ll be amazed how much you get done when you turn off your friends.


But of course this works only for web sites, not for local applications.