Will McCarthy, in a recent article in Scifi Weekly, says that if we wanted to, we could make the One Ring that Frodo went to so much time and trouble to toss into the fiery pit from whence it came:

Many people write to me asking, "Hey, Wil, couldn't we make one of those One Ring thingies using technology, and rule the world in an orgy of stupefying eeevyil?" The answer, happily, is yes.

The most notable characteristic of the One Ring is that it turns its wearer invisible. . . .NASA and the U.S. Air Force have already experimented with "video camouflage" which places cameras on one side of an object and video screens on the other side. The object "disappears" before our very eyes, like the alien trophy-hunter in Predator, and with the hoped-for advent of projective holography, this technology can only improve.

The One Ring also seems to extend the lifespan of its owner, while causing gradual behavioral changes such as irritability, light-sensitivity, modified diet and megalomania. . . .This effect is consistent with various forms of heavy metal poisoning, combined with a slow release of the chemical superoxide dismutase (SOD), which has been implicated in the long lifespans of certain genetically modified worms.

Of course, McCarthy goes on to say, if we were to make an object with the One Ring's capabilities using modern technology it would likely weigh in at several hundred pounds and be way too big to wear on your finger. It would be better incorporated into something like, say....a Humvee.