And Popular Mechanics editor Mike Allen, in a report called "The Truth About Water-Powered Cars: Mechanic's Diary," described the water-powered car kits advertised on the Internet as "rubbish" and "outrageous." Only under tightly controlled settings, Allen wrote, might a hybrid HHO and gasoline car see modest increases in fuel efficiency.

In other words, claims by some sites that these kits will increase your engine's gas mileage by 10 to 80 percent are not realistic.

Free Gas Cards: Not Such a Deal

I found one typical free-gas offer by clicking a search-engine text ad with a pitch that read, "Getting A $1000 Gas Gift Card For Free Is Simple," for a site called ExxonMobileGiftCard.com. I didn't pursue the offer, however, because my McAfee SiteAdvisor browser toolbar identified the hyperlink presented within the ad as "unsafe" and described the site it links to as a "high volume or spammy e-mailer."

It turns out that the McAfee SiteAdvisor warning didn't even apply to ExxonMobileGiftCard.com: SiteAdvisor spokesperson Shane Keats explains that the McAfee warning actually applies to a site called paydayasap.com to which the link sends you (transparently) before redirecting you to ExxonMobilGiftCard.com. SiteAdvisor flags PayDayASAP.com: because people who signed up for its promotions received nearly 150 unsolicited e-mail pitches within a week.

While stressing that the ExxonMobilGiftCard.com site (which does not appear to have any relationship to ExxonMobil) has not been formally reviewed by SiteAdvisor, Keats says the site "has all the hallmarks of a scam site."

Keats says most "free gas" sites, like free iPod sites, are really just come-ons designed to make money from visitors. ExxonMobilGiftCard.com, for example, pitches you dozens of services, products, and free trials (including an "Ivory White Teeth Whitening free trial kit" that requires a credit card payment of $4.87 for delivery).

The site requires that you take what it calls a survey, which is basically a series of sales pitches for weight-loss pills, car loans, books, and the like. After you've declined all of them, you're told that to get the gift card you must accept at least two offers.

Keats says that many of the "free gas card" sites SiteAdvisor has reviewed earn commissions on the offers--and that if you share your e-mail address, phone number, and maybe even cell-phone number, you can expect a deluge of marketing e-mail messages. The value of the commissions and your personal information can easily exceed that of the free gift card.

And you may not ever see the card: Keats says less than 5 percent of people who sign up for every offer actually get the "free" gas card (or iPod).