Is there a application that limits time spent online?

[completeunknown91]

I would like to limit the hours that I use social networking sites, as I think this would make me more productive. I currently have a preference on Firefox that prevents me from opening more than 5 tabs. Is there such an application that you can set parameters for when you can go on certain websites. For example, if I wanted to block Facebook, is there a program that would let me do that from 11am-1pm?
I think something like this would really put you in the right habit.