Mac OS X only: When it's time to dig deep and do some serious focusing on a task—and refrain from surfing or checking email entirely for a block of time—you want temporary internet disconnection utility Freedom. Freedom serves a simple purpose: It disables all wireless and Ethernet networking on your Mac for up to six hours at a time. After the time you specify is up, Freedom re-enables your network adapters and display a confirmation. What, think you can just turn Freedom off to hop online to check the Olympic medal count? Not so fast, buster.
From the Freedom Readme:
Stopping or quitting Freedom will not re-enable your network adapters. This is purposeful. To re-enable your network before the time period elapses, you must restart your computer. Alternately, if you are skilled at UNIX you can override Freedom without restarting.
The point is to make it difficult for your internet-addicted self to override your sensible "must focus" self—much like the LeechBlock Firefox extension. Freedom is a free download for Mac only.