A Google Chrome plug-in that allows the blocking of custom time-wasting websites, with on/off functionality. Click on the W in the top-right toolbar to enable Workmode, use the options page to denote which websites to block! I personally use it all the time. Coded by Shreyas Tallamraju.
NEW UPDATE (v3):
~~Custom Blocks + Chrome Storage API Usage~~ Now, custom blocking is now available! Click on the Options page of WorkMode (Right-Click on icon > Options) and you can now choose which websites are blocked!