What started as a little event in Sydney in 2007 has grown to a massive worldwide event involving more than 4000 cities.
In an effort to raise awareness about carbon emissions, householders were asked to turn off all lights and electrical appliances for one hour on March 28, starting from 8:30pm (the time chosen because that's when it's finally becomes pitch black in our summer daylight savings time).
I remember participating in this in 2007. It was quite exciting and I remember sitting in almost darkness with candles suddenly aware that time seems to stand still when you're without light (no tv, no internet, no reading, just talking or listening to music).
In 2008 I didn't do it as I wasn't in the country, and last year (2009) I remember playing Pictionary in the (almost) dark with some friends who came over! This year (2010) I was at a friends house and we only managed to last 20 minutes before we got 'bored' and decided to play their new console games (with the lights off mind you, so we were half doing it, OK, half cheating too ;) ).
I'm pleased to hear that Paris got in on it too :)
Sydney Morning Herald report
Earth Hour site
Le monde : Earth Hour