Girdle of femininity/masculinity

The Girdle of Femininity/Masculinity is a type of magical item in the Dungeons & Dragons fantasy role-playing game which is infamous for its unusual effect.

Publication history
The "girdle of femininity/masculinity" first appeared in the original 1979 Dungeon Masters Guide, detailed on page 145.

Description
Such a girdle looks like an ordinary leather belt, but when worn immediately switches the wearer's sex to the opposite gender, then loses all power. Additionally, 10% of these items remove the sex of the wearer. The change causes no actual damage, but it is permanent and most characters can be expected to be very uncomfortable with it.

The Girdles are included in the first and the second editions of AD&D, their magic potent enough to be fiendishly difficult to reverse; even a Wish spell has even odds, though a deity can set things right. They appear to be absent from the third edition, though similar effects are mentioned as a possible curse outcome.

Using a Girdle is the best-known if not only method to bring about such an effect, and over the years they have served to bring mirth, cheap jokes, and role-playing opportunities to D&D fans everywhere.

"Reverse user's gender" is also one of the random cursed item effects in the 3rd edition of D&D.

Other media
One such item also makes an appearance in Baldur's Gate, where it's quite possibly among the first magical objects the player finds, but only takes a Remove Curse to do away with.

In popular culture
The webcomic The Order of the Stick introduces a Girdle early on and brings it back much later, where it's used to good effect and later yet undone with a Remove Curse. This, combined with the fact that it is taken from an ogre, suggests that it may be a reference to the one found in Baldur's Gate.

One of items in NetHack is Amulet of Change, which has the same functionality. It crumbles to dust when worn, preventing any kind of reversal other than finding another such amulet or having luck with polymorph.