http://mas.org/programs/tributeinlight/
http://en.wikipedia.org/wiki/Tribute_in_Light
Tribute in Light was first presented on March 11, 2002, six months after the attacks, and MAS has presented it annually since. Comprising eighty-eight 7,000-watt xenon light bulbs positioned into two 48-foot squares that echo the shape and orientation of the Twin Towers, Tribute in Light is assembled each year on a roof near the World Trade Center site. The illuminated memorial reaches 4 miles into the sky and is the strongest shaft of light ever projected from earth into the night sky. See this list of great Viewing Locations.
It was independently conceived by several artists and designers who were brought together under the auspices of MAS and Creative Time. Tribute was designed by John Bennett, Gustavo Bonevardi, Richard Nash Gould, Julian Laverdiere and Paul Myoda with lighting consultant Paul Marantz. It was originally made possible by a grant from the Lower Manhattan Development Corporation and with the generous assistance of Con Edison.
See and listen to the origins of Tribute in Light and how it is produced annually on September 11.