None of these were made up by me...

Coin of False Fates: When flipped and called in the air, it always lands on the opposite side called.

The Automatic Signpost: whenever this ordinary looking wooden sign post hears the name of a City anywhere on the same Plane, it spins around a few times and ends up pointing in the direction of that City, with the travel times for walking there, riding a coach, boat, airship, and/or any other travel device available to your setting.

Cloak of the Mysterious Stranger: A full-length heavy black cloak, which is always flowing in a mystic wind. The wearer's face is always shrouded in darkness, and the wearer is always outlined in a looming shadow, regardless whether there is any light source nearby or not. Also, a narrow sliver of light plays across the wearer's eyes, making them stand out dramatically against the darkness of the cloak and his shrouded face.
Whenever the wearer moves, there is a swooshing noise, pnctuated by the snapping of the cloak in the wind (whether or not there's any actual wind). The wearer's footsteps create a dramatic echo (booming, rapping, or crunching, as the wearer prefers), no matter what surface is being walked on, or any attempts to be silent.

The Book of Tales: This ancient, leatherbound tome has no writing within it, its pages all completely blank. But should you write a person's full name into the book the pages magically fill with the complete story of that person's life, no matter who they may be or when they lived. Upon next being closed, the book's pages become blank once more.

The cloak is my favorite, it was used to great effect in a campaign that I played in.