Of all the fancy-shmancy apps that I use every day to do whatever it is that I do, SelfControl is probably one of the coolest, simplest, and most important. All it does is block you from visiting distracting sites for a fixed amount of time when you should be working. The app’s site explains it best:

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, by deleting the application, or by restarting the computer – you must wait for the timer to run out.

I would literally get nothing done ever without it. Go pick it up FOR FREE here.