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SelfControl
Because sometimes you don't have any.
Are your favorite websites or your emails a distraction? SelfControl is an OS X application which blocks access to mail servers or websites for a predetermined period of time. For example, you could block access to your email, Facebook, and Twitter for 90 minutes, but still have access to the rest of the web. Once started, it can not be undone by the application or by restarting the computer – you must wait for the timer to run out.

The main SelfControl page is
here. Or download the application here.

SelfControl was developed by Charlie Stigler for
Steve Lambert, a fellow at the Eyebeam Art and Technology Center. SelfControl is free and open-source software, released under the GPL. The source code is available at Github.

Media:

Wired
Macworld
Ars Technica
Lifehacker
Digg
Slashdot
Tranquil Apps (a nice writeup on trying to beat the block)