The other day I changed my Linux window manager to stumpwm (also tried ratpoison but stumpwm works slightly better for my particular case). It's definitely helped me focus, and the fact that I have to run a command or keystroke to get to the Internet gives me just enough extra effort that I can stop myself.

However, I also find it somewhat grueling to go without any distractions for a long time. After about 4 hours straight I'm surprisingly drained.