WasteNoTime is a new Safari Extension which allows you to block those time-wasting sites that can suck the life out of your day. All you need to do is to specify which sites to block and when to block them. You may add sites to be blocked in batch mode or do it while you are browsing. And you can configure the maximum amount of time you should spend on browsing each day, within and outside work hours separately. You may define multiple time ranges to match your work habits.
This extension is similar to LeechBlock extension on FireFox. It can be downloaded through Apple Safari Extension Gallery or WasteNoTime website.