Herold, 32, makes four trips a year to his hometown of Boston, at times paying as much as $400 for plane tickets.

He found he could save money by comparison-shopping with SideStep (http://www.sidestep.com), paying just $216 for a recent round trip he booked with the travel search engine.

"They haven't let me down yet," he said.

Not all online buyers have found such bargains, but they are turning increasingly to comparison-shopping engines when they're considering a purchase of everything from baby items to tires.

Last month, 46.6 million Internet users -- or almost one-third of the total U.S. Web audience -- visited a comparison-shopping site, according to Nielsen//NetRatings (NTRT.O: Quote, Profile, Research) .

To put that number in perspective, online dating sites received about half as many visitors in April, the audience measurement company said.

New innovations are making it faster for online comparison-shopping services to gather data, and more retailers are participating in them as they become more popular.

As a result, comparison sites that once catered only to techies in search of rock-bottom prices on laptop computers, digital cameras or other gadgets have grown broader and more sophisticated.

As of April, Shopping.com (http://www2.shopping.com) was the market leader followed by BizRate.com (http://www.bizrate.com) and NexTag.com (http://www.nextag.com). All offer a broad range of mainstream products like blenders and baby car seats as well as electronics.

Not to be left out, the big Internet services are also in the mix.

Yahoo Inc. (YHOO.O: Quote, Profile, Research) , which runs the most-visited online destination (http://www.yahoo.com), recently bought European comparison-shopping engine Kelkoo (http://www.kelkoo.com) amid efforts to beef up its Web search and shopping services.
On other fronts, Silicon Valley-based Yahoo in March relaunched its Autos channel (http://autos.yahoo.com), where users can compare up to four cars on price, features and performance -- something many stand-alone engines cannot provide.

Google Inc. recently added a link to its Froogle comparison site on its main search page (http://www.google.com).

According to Nielsen//NetRatings, the service got more visits in April than older rivals PriceGrabber.com (http://www.pricegrabber.com) and mySimon (http://www.mysimon.com), even though it is still in test mode.

Among specialists, SideStep's downloadable tool allows travelers to search dozens of sites for real-time prices on hotel rooms, rental cars and flights -- including those on low-cost carriers like JetBlue (JBLU.O: Quote, Profile, Research) and Southwest Airlines (LUV.N: Quote, Profile, Research) .

Because it pops up when users are searching for travel-related items, SideStep also can be used to do side-by-side price comparisons.

APPLES TO ORANGES

Shopping.com has filed for an initial public offering and other segment players are mulling a similar move as they bask in their popularity and accompanying hype.

Part of their success has come as large retailers realized that comparison sites can drive significant traffic to their own online stores while sparing consumers the hassle of having to jump from site to site in search of a deal.

Even so, the industry's biggest players are still missing out on some deals. For example, Shopping.com recently showed a Graco ComfortSport car seat from discount retailer Target (TGT.N: Quote, Profile, Research) (http://www.target.com) for $79.99, but did not have a price from Wal-Mart (WMT.N: Quote, Profile, Research) , whose site (http://www.walmart.com) was selling a model for $78.74. Still, the lowest price was $67.99 from Baby Universe (http://www.babyuniverse.com).

That example also underscores a common consumer complaint about the difficulty of making apples-to-apples comparisons because of such factors as differing model numbers or price quotes that exclude shipping and other costs.

As with many other commercial Web services that track information about user habits, comparison-shopping sites' privacy policies have sometimes ruffled feathers.

While Shopping.com has received praise from consumer privacy advocates because it gets users' permission before sharing information, some consider SideStep's technology spyware because it tracks what users do on the Web.
"We hold any information you provide with the utmost care and security, and will not use it in ways to which you have not consented," SideStep said on its Web site.

Meanwhile, some consumers find they need to look beyond the price-comparison sites before they make their purchase decisions.

Jennifer Blake, a former marketing executive at a Northern California-based retailer, tried out some sites in her hunt for a new car seat for her 10-week-old son, who is about to outgrow his infant model.

Blake, a self-confessed Consumer Reports junkie, said that in this case she found Shopping.com's Epinions user reviews less helpful than safety test data from a respected source.

"What one mom says about how easy it is to use isn't as important as the safety," Blake said.