iButton, secure, very robust, small and easy to use

There are a lot of technologies available, like smart-cards, barcodes, RFID tags, magnetic stripe and others.
The iButton is more robust than any of those technologies, can perform different tasks, like keeping track of temperature or time, store ecash or other data, is small but still user friendly and much more secure.
The silicon chip inside the iButton is protected by one of the most durable materials: stainless steel.
You can drop it on the floor, scratch it, wear it even when you go to swim or simply keep it in your key chain: it has been wear-tested for ten-year durability.
No other identification device can do the same.