Ring of teleport control
Wearing a ring of teleport control (or ring of TC) conveys the teleport control extrinsic. This is an incredibly useful property, especially when combined with teleportitis. Eating a ring of teleport control grants the property as an intrinsic.
One of the ways to identify a ring of teleport control is to teleport (usually with a teleport trap or scroll) while wearing an (ideally noncursed) ring. If you can choose your destination, you have the teleport control ability. Another method (if you lack the means to teleport) is to wear the ring and attempt to teleport with ^T; you may be told "You are not able to teleport at will" instead of the usual "You don't know that spell."
Dropping a ring of teleport control into a sink produces the message, "The sink looks like it is being beamed aboard somewhere."
