1) Unidentified lights in the night sky in WWII seen by some American fliers over the Pacific Ocean and Germany at night. "Foo" comes from a comic by Bill Holman. Not fully explained, but most likely a result of "Aviator's Vertigo."
2) A great, though uneven, rock band fronted by Dave Grohl, who may be the coolest thing that has ever been.
When flying over the Atlantic, Dave Grohl reportedly saw some foo fighters, and said "cool." Then he went back to sleep to dream about how cool he is.