A short piano chord alert noise installed by default on Macintoshes. It's actually named that way because of a spat with Apple Records, the record label of the Beatles. Apple Records sued Apple Computer for making playback-capable computers, seeing this as trademark infringement. Apple Computer promised that they would never sell computers that could record sound, and when they introduced a system capable of recording (I believe it was the Macintosh SE), they made this beep in homage.

Apple Records became once again angry with Apple Computer in 2003 because of the iTunes Music Store.

I briefly remember reading about this in David Pogue and Joseph Schorr's Macworld Macintosh SECRETS, an excellent book full of wit and wisdom for all you Mac types.