Why hand over your credit card to cashiers when they could scan your plastic while it's still in your wallet? Or what if you could make a purchase by waving your hand—with a chip implanted just beneath your skin—over a wireless reader?

That's the promise—or threat, depending on your point of view—of RFID, a wireless ID technology that's spreading faster than you might think.

If RFID (or "radio-frequency identification") sound like the stuff of science fiction, consider the fact that the EZ Pass tag on your windshield—the one that lets you fly through highway toll booths—have RFID to thank for their abilities.

RFID chips: They're everywhere!

RFID tags are also found in passports (if you were issued a U.S. passport in the past year or so, chances are there's an RFID chip in there), library books, jewelry, credit cards, and even garbage trucks.

Think of an RFID tag as a barcode—except instead of a pattern of lines that must be scanned up close, RFID tags actually transmit their data to readers anywhere from inches to hundreds of yards away (the range depends on the chip—and use—involved).

Smaller, "passive" RFID tags are tiny enough to fit under a sticker or actually be implanted (ouch!) beneath the skin.

More complex "active" RFID tags, on the other hand, come with their own power sources, have longer ranges (hundreds of yards, in some cases), and can take readings on temperature, humidity, and other environment factors—an ability that comes in handy when it comes to tracking, say, perishable goods such as produce.

Inventory tracking, made easy

Indeed, RFID tags are making headway in the field of inventory control, allowing vendors (such as Wal-Mart and Rite Aid, among many others) to instantly track their stock—and in some cases, detect if goods have been exposed to harmful temperatures, vibrations, shocks, or even light.

Same goes with library books. Using the latest RFID tags and readers, librarians can scan and track entire stacks of books at once—a much less time-consuming prospect than pulling out individual volumes to read their barcodes.

RFID tags can be good at tracking people—too good, if you ask some privacy experts.

In addition to being embedded in passports and credit cards, RFID tags have been sewn into school uniforms to track children, implanted in hospital patients to prevent accidental treatments (although only a handful of patients have agreed to be "chipped"), and even swallowed (ewww!) to track the effectiveness of medication. And get this—clubs in Scotland and Spain have chipped party-goers who'd rather not bother with paying cash for drinks.

Hold still, this won't hurt a bit

Naturally, the idea of humans with RFID tags implanted in their skin leads to fear of Big Brother, tracking our every move—not to mention the fact that surgically implanted chips are, by their very nature, creepy.

There are also plenty of security issues to consider—namely, what if someone managed to sniff out your passport's RFID tag and swipe your identity? Or your credit card number?

In fact, as RFID tag technology has improved, so have covert RFID readers, which can read at a distance RFID tags that were designed to work only at close range (such as those in passports).

Hackers are also getting quite good at cracking the encryption in RFID-enabled smart cards, as students and researchers at the University of Virginia recently proved.

Meanwhile, health concerns about human RFID implantation have cropped up, with some studies showing cases of chips causing tumors in lab animals. The FDA approved human RFID chipping back in 2004.

What’s your take? Like the idea of being able to buy your groceries with a wave of your hand? Or does the idea of an RFID implant make your skin crawl?