The movement was started last year in Sydney but this year it is extending to the world. Everybody all around the world is taking part, even the businesses/corporate.
More information about the Earth Hour here.
UPDATE: Melbourne city will also be dark during Earth Hour with most of the businesses committing to participate in the event. Restaurants that are 'switching off' will have candle lighting up the tables. This might be a rare chance to see what Melbourne looks like with no lights. Of course there are other cities that will participate in Earth Hour, full details go to the website.