Before her big kidnapping (see "Police Reports," above), Idunn gets a few other mentions in the Prose Edda, most importantly as the bearer of the immortality-bestowing apples. Later, we learn some of her rockin' nicknames (see "Profile").
This poem precedes Balder's Dream in some later manuscripts of the Poetic Edda. Although she may not play a big role in the rest of its plot, Idunn's initial fall from the world-tree is the ominous omen that prompts Odin to send a group of gods to the underworld in search of prophecy from a wise-woman. We also learn here of a possible Elvish/dwarvish parentage for Idunn. She sure is a mysterious lady.
In this opera, the first of Wagner's four-part Der Ring des Nibelungen, the character of Freia is a combination of Idunn and Freyja. So much for accuracy. As a result of trading Freia to the giants in exchange for a castle, the gods lose her apples of immortality and begin to age.
In the Marvel Comics universe, Idunn appears as part of the Thor series starting in 1965, with her function as bearer of immortality-bestowing apples largely unchanged. Way to stick with what you know, Idunn.
In this role-playing game for Xbox 360, the Norse pantheon is reinvented as futuristic humans. Idunn is an important "cyberneticist" whose apples are "nanotechnology and molecular enhancements" that make the humans nearly immortal. Well, then.