Is email a distraction? SelfControl is an OS X application which blocks access to incoming and/or outgoing mail servers and 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.
Free software under the GPL. Here’s the code.