Get Shit Done is a command-line utility that blocks popular, known-to-be-distracting websites so you can get your work done. Even if you're not a command line pro, it's pretty simple to use and blocks time-sucking sites without fail.
To get it to work, all you need to do is download the source from GitHub, open your command line app of choice, switch to the directory containing the script, and execute the following command:
sudo get-shit-done work
When you're done, use this command:
sudo get-shit-done play
You may need to edit the get-shit-done.py script in a plain text editor if you're having trouble, as it contains references to your host file and other variables — like the sites that get blocked — that you may want to edit. When running it on Mac OS X, I didn't have to make any alterations but it did throw an error each time it ran. Despite that, it worked just fine.
Get Shit Done [GitHub]