Dallas Semiconductor/iButton
The iButton™ is a 16mm computer chip armored in a stainless steel can. Let's start with the package. Because of this unique, durable package, up-to-date information can travel with a person or object. The steel button is rugged enough to withstand harsh outdoor environments; it is durable enough for a person to wear everyday on a digital accessory like a ring, key fob, wallet, watch, metal card or badge (all of which, incidentally, you can purchase from this site).
Now let's move to the computer chip inside the package. There are multiple different iButtons available. Each starts with a guaranteed-unique registration number engraved in the silicon.
The iButton is ideal for any application where information needs to travel with a person or object. Affixed to a badge, key fob, watch, or ring, an iButton can grant its owner access to a building, a PC, a piece of equipment, or a vehicle. Attached to a work tote, it can measure a variety of processes to improve efficiency, such as manufacturing, delivery, and maintenance. Some versions of the iButton can be used to store cash for small transactions, such as transit systems, parking lots, and vending machines. The iButton can also be used as an electronic asset tag to store information needed to keep track of valuable capital equipment.