BNOF is an acronym for "Blue Note Of Freedom", which refers to Guitar Hero 2, more specifically the song "Jordan" by Buckethead. The BNOF occurs right after the hardest part of the guitar solo in the song, and regarded by most as the hardest solo in the game. Reaching this note almost always indicates a pass of the song, which is an incredible feat.
"I failed right before the BNOF!"
"60% when I failed, I could see the BNOF!"