In London, when you travel by Tube, DLR, ferry, bus or tram and some overground trains, you will usually use an Oyster card, a blue credit card-sized piece of plastic which you load with money and then touch against yellow contact points when boarding (and usually when leaving) these services. I believe the system works like this: when you hold the card within a few centimetres of these large-ish, yellow contact points, a magnetic field from the reader induces a current in a rectangular wire antenna which is inside of your card. This in turn powers the chip in the card and broadcasts some data to the reader which then gets processed or some such.
The exact mechanics of what happens after the current is induced aren't relevant. My idea is that you can turn the Oyster card from a flat rectangle of plastic (with some miniature electronics embedded in it) into a ring that you wear on your finger. Put the chip in the ring and run the antenna around the circumference a few times.
Instead of having to keep hold of the card, in your wallet or in your pocket, you just leave it on your index finger semi-permanently. I don't think this makes the Oyster ring significantly more difficult to forget than the Oyster card - you would still have to remember to put it on again when you leave the house every morning, just like you have to remember your wallet or whatever you keep your card in usually. The clever bit is this: by not actually physically carrying the card, but just having it attached to your finger, you free up a hand. An entire hand! Think of the many additional things you could carry on the Tube if you didn't have to hold an Oyster card in it as well. Imagine how much easier struggling through the gates with both hands full of luggage would be!
Imagine how cool it would look to walk up to the entrance to a Tube station and pass the gates with nothing but a wave of your hand.
Now, my electrical engineering is spotty, but I believe that the induced current is dependent on the cross-section of the antenna. Because the cross-section of the ring is much smaller than the cross-section of a credit card, less current would be induced and the chip might not receive enough power. However, I think this could be solved by running the antenna around the ring multiple times.