You need this! I did and I didn’t know it. Fred has released Freedom. Freedom isn’t free — first you must have an Apple then you should donate $10 to Fred. He says:

Freedom is an application that disables wireless and ethernet networking on an Apple computer for one hour at a time. Freedom will free you from the distractions of the internet, to allow you time and space to code, write, or create.

Many studies have found that hour-long chunks provide optimal work flows. At the end of one hour, Freedom re-enables your network, restoring everything as normal. Freedom enforces freedom. If you quit Freedom, you will need to restart your computer to re-enable networking.

UPDATE: Fred explains Freedom further on his blog.