[by analogy with the toyboxes on theater organs]
n. Features added to a program or system to make it more flavorful from a hacker's point of view, without necessarily adding to its utility for its primary function. Distinguished from chrome, which is intended to attract users.
"Now that we've got the basic program working, let's go back and add some bells and whistles."
No one seems to know what distinguishes a bell from a whistle.