We have but one item here at the moment, but in time more may be added.
The Orb featured in one (rather bad) campaign I ran, and in a different form in a one shot game. In the campaign, it was a device that allowed random interdimensional travel. It really was just a plot device to allow for whatever kind of adventure I felt like running. In the one shot, it was a device for interdimensional summoning that allowed an evil wizard to summon Cthulhu Mythos monsters into a basically RuneQuest setting.
The Orb is an ancient artifact either created by deities or a powerful wizard. Depending on how it is used, it can allow extra-dimensional travel or aid in summoning things from other dimensions. The trick is that if you don't know how to use it, you'll go to a random place or summon a random creature. It is probably unique, and of great value to wizards and possibly adventurers. The adventurers in the campaign didn't know how to use it, thus travelling randomly, and the wizard used it with an arcane tome to summon and control monsters. When used to travel, it does have the safety feature of always returning the travellers to their starting place. It looks like a large pearl about one foot in diameter, and is usually placed in some kind of mounting or rack.
When used in campaigns, it probably should not come with instructions. More interesting that way; just have a few (possibly inaccurate) legends about the thing.