Self Control is a great, free app for Mac that allows you to block website that are distracting to you for set amounts of time. Say you are having trouble concentrating because you can’t stop checking your Gmail account. Then add www.gmail.com to your black list and the app will not allow you to access the site for the time that you set. You decide what is blocked and for how long. Here is a video tutorial by one of my students explaining the features of the software:

You can download Self Control here.