Developer website: Benjamin Dobson
Earth Hour... Stop yourself from using your computer during Earth Hour with this free application. Simply double-click on it and leave it to alert you at 20:20, 20:25 and block your screen at 20:30. The app will automatically quit at 9:30. Apologies if it doesn't work... I threw it together this afternoon and haven't had much testing time.
Requirements
Mac OS X 10.5 or later