Maybe in the long term the right answer for dealing with Internet distractions will be software that watches and controls them. But in the meantime I’ve found a more drastic solution that definitely works: to set up a separate computer for using the Internet.
I now leave wifi turned off on my main computer except when I need to transfer a file or edit a web page, and I have a separate laptop on the other side of the room that I use to check mail or browse the web.
The problem with this, however, is that some people need to be connected to the Net to work. At Blend, we have to always be connected to our dev servers. To make this work, everyone would need to work locally, which may or may not work.