The second annual observance of Earth Hour, is scheduled world wide for 8 to 9 PM tonight (local time, not simultaneously), Saturday March 29. Cities, communities, corporations, and individuals around to globe are being asked to voluntarily turn off all the lights, TV, stereos, computers, microwaves, and other electricity consuming accouterments of modern life for one hour.
The idea is for each city/community to cut the power at their own time zone's 8:00 PM, so that there is an effect of rolling darkness following the setting sun.
The first Earth Hour in 2007 was a project of the World Wide Fund for Nature Australia and the Sydney Morning Herald. More information is available at the Earth Hour - North America web page and The Daily Kos (where there are some nice comments about thing people might do instead). .