Begin with one of the most haunting images on the web: Flightexplorer, a company that tracks commercial flights in the air, posts this radar map of American Airlines Flight 11's path from Boston to the World Trade Center. Where does the disaster begin? With the sharp left turn the plane takes over the Adirondack Mountains.
The Slate home page posts all the stories we've published on the attacks.
Many Web news sites have been overwhelmed with traffic, but you should be able to view MSNBC's comprehensive coverage here.
MSNBC lists emergency numbers you can call for information about airline passengers and some World Trade Center employees. United Airlines will post updates on the disaster here. American Airlines will post them here. Morgan Stanley, which employed 3,500 people in the World Trade Center, posts its sad message here. You can check the American Red Cross site for information about donating blood.
The World Trade Center and the Pentagon:
The World Trade Center's site is, unsurprisingly, down. This is the most recent version of it cached by Google. This page, maintained by someone who must have had a World Trade Center office and a Webcam, offered real-time images of the Hudson River. Now it is a ghostly black. Worldtradecenter.com is nothing but this single grim sentence: "Domain to be donated as memorial or for some other suitable purpose."
The Department of Defense's Pentagon site includes a history of the building and a virtual tour.
TerraServer posts satellite photographs of lower Manhattan and the Pentagon. It sells photos of those sites, or anywhere else, to anyone who wants to buy them.