The Wand of Wonder
This little puppy is the random hilarious story generator for D&D. It's use results in random effects each time it is used, using my favorite of old-school charts: The percentile effect chart! One of the best things about this Magic Item is that any character class can use it (in 3e+ editions it is known as the "Rod of Wonder"), and any GM worth his salt will make the activation of the wand either written right on it or better yet make a big red button right on the side of the darn thing, begging to be pushed. The point is to make it so anyone can use it as soon as its found, and allow them to completely muck up/save a perfectly normal combat by doing something ridiculous.
The reason I prefer the Wand of Wonder over the more infamous Deck of Many Things is that the Wand isn't a campaign-killer. The Deck of Many Things has so many powerful cards that violently swing between summoning demons to kill the party or giving the party godlike-powers that once it is present in a campaign then you know the campaign is about to end, for better or worse. The Wand of Wonder gives the party a good amount of random effects that can be quite powerful, but aren't game-breaking. The stories of using a Wand of Wonder are frequent and fun, and never end with "So then I drew a Jack of Clubs..."