Java Ring:

I worked @Sun for ~15 years. I was part of a team that built some of the very first apps for demo'ing the capabilities of the Java Ring, including an interesting demo where we created our own (toy) crypto java currency (to make it easier to demo, we built in the ability to "mint" new "coins" for anyone who had a ring and wanted to try the demo - not very secure, and hyper inflationary), stored it on the ring, and used it to buy cans of coke from a coke machine (embedded the reader into the payment receptacle on the vending machine). The software was easy, the hard part was going from digital to analog and back by integrating the reader into the vending machine (this was pre-digital vending machines - we had to rewire the whole machine, learned a ton about how multi-step electro-magnetic solenoids work). For the digital piece we used an embedded PC running Linux (and Java), with wireless Ethernet.

I think I still have several rings lying around somewhere - the readers were cheap, but the rings were pretty well made.

- DeveloperFish