A Wand of Wishing is the most powerful non-unique item in
Nethack, since it alone is a surefire
* way to be granted a
wish, free of the risk of a perilous
backfire and usable repeatedly. A
wish in turn is a chance to create practically any item, including
the strongest armors and
most powerful spellbooks: a great
boon to any character. A supply of several can create most of an
Ascension Kit and
turn the tide of the game. One wand is mercifully (
!) guaranteed to be hidden on the
Castle level, right above
Gehennom and the earnest start of the
real action.
There are a few tricks to using the Wand with maximium efficiency. A Wand of Wishing is generated with 1-3 charges, and can be recharged once - subsequent attempts will cause it to explode. A blessed charging fills it back to three, so unless a user already has the means the first wish should be for them. A blessed scroll of charging is worth wishing for if its type hasn't been identified, a blessed Magic Marker if it has, as the latter can then be used to inscribe the former with enough power left for some scrolls of genocide. Inscribing a scroll of an unknown type is unlikely to work, save for wizards with maxed-out luck (and are you really certain that yours is?). Applying an empty wand far too many times will wrest one last charge out of it, making the total 5-7 wishes. It's not possible to wish for more.
Plot-related items - all four plus the Quest artifact - can't be wished for. Other artifacts may or may not appear, depending on the number of artifacts already in play. One is marvellous, seven isn't. Wishing for other classes' Quest artifacts is possible, but most spell trouble for those not in their class and all for those not in their alignment. Caution is advised.
Modifiers, such as blessing, greasing and proofing, can and should be added freely. Stackable items should be asked for several at a time and enchantable ones at more than +0 - it may succeed and won't cause harm. The odds are again inversely propotional, making the best place around two copies or +2.
Finally, players attempting to go back to fix a typo have been known to find that they've fumbled and lost the wish. It's not a pretty sight. I recommend staying away from the The Staff of Aesculapius.
Sources:
Devouring about half of the Nethack spoiler files on the Internet, definitely including these:
http://nh.gmuf.com/wish.html
http://www.steelypips.org/nethack/341/art1-341.html