1- Self Control
SelfControl is a free and open-source application for Mac OS X (10.5 or above) that lets you block your own access to distracting websites, your mail servers, or anything else on the Internet. Just set a period of time to block for, add sites to your blacklist, and click "Start." Until that timer expires, you will be unable to access those sites--even if you restart your computer or delete the application.2- Anti-Social
Anti-Social blocks the sites that make you unproductive. Start Anti-Social and Facebook and Twitter go away.
This is a Chrome extension ( also available for Safari and Firefox) that increases your productivity by limiting the amount of time that you can spend on time-wasting websites.
Freedom is the wonderful app for Windows and Mac computers that locks you away from the 'net. Turn Freedom on, tell Freedom how long you'd like to focus, and you're offline and able to work. If you need to cheat and get back online, you can reboot.