Asiana Airlines Flight 214, a Boeing 777, crash landed at San Francisco Airport on Saturday, July, 2013 with 307 passengers and crew on board. As of this writing (1:19 PM PT) there are no confirmed fatalities, but it's too early yet to give a final accounting. The plane did not report any problems before landing, and caught fire on the ground.
SFO airport was closed for several hours, and flights were diverted to other area airports.
This is breaking news, and here are some sources for it as the story unfolds.
Wikipedia: Asiana Airlines Flight 214
Asiana Airlines: Press Release for Incident Involving Asiana Flight OZ 214 (July 7, 2013 06:30 KOR.Time)
Passenger David Eun tweeted from the tarmac minutes after the crash.
Newspaper reports: The Guardian, Boeing 777 from Seoul crashes on landing at San Francisco Airport. New York Times, 2 Die and Many Are Hurt as Plane Crashes in San Francisco.
Reddit: Apparent 777 crash at SFO.
A good technical discussion of the aircraft is on Airliners.NET: OZ 777 Crashes at SFO. See also Flight Aware, Asiana 777 (AAR214) crashes upon landing at SFO; Aviation Herald, Accident: Asiana B772 at San Francisco on Jul 6th 2013, touched down short of the runway, broke up and burst into flames
UPDATE: Passenger/crew count updated to 307 from 303 per press conference at 7:45pm PT.