C-flat, it's not uncommon in rock at least - it allows the singer another semitone of range, which can be useful. Oasis for one tend to do that.

Tin whistles come in C and D tunings as well, where you play the same fingers on the C but it comes out a tone lower (again IIRC, my wife's the whistle player, not me :-) I guess this would be to fit in with melodeons etc tuned the same, as per Greg's post.

Graham.