This was the field in which Flight 93 crashed on September 11, 2001. The passengers and crew, having realized the nature of the terrorists' plan, attempted to retake control of the aircraft. During the ensuing struggle, the aircraft crashed. The attempt, even though it ended in failure, succeeded in keeping the terrorists from using the aircraft to kill hundreds more.
Update May 7, 2009: While I was at this site, I overheard people talking about the Government trying to negotiate with the landowners to acquire the land for a permanent memorial. As I understood it, the landowners were allegedly holding out for more money.
In a news article I ran across today, it turns out the Government has decided to appropriate the land.
I understand that, from one perspective, this feels like a violation of property owners' rights. But I have to say that in this circumstance, I agree completely with the Government. It had to be done.