From Seeplain DND:
Save Time by Blocking Out Addictive Websites and Programs from your computer when you need to get work done. Simply select which Websites or Programs you get distracted by and set a duration of your choice. DoNotDisturb will then Block Out your selection and you will not be able to access these addictive Websites or Programs until the time you have set is up. It stops you wasting valuable hours on addictive Websites and Programs such as Social Networking Sites, Instant Messenger Chats, Computer Games, and Video Clips. Version 2.3 adds reversal feature that allows the selected websites and programs while blocking everything else, adds Warning Only feature that does not block the selection but warns when accessing barred websites and programs.