Think of it as the online equivalent of the entire city of Omaha converging on a single department store. Back in August, the big retailer Gap decided to go where few large retailers have gone before—using a popular discount Web site to hold a nationwide sale. Shoppers had 24 hours to go online at Groupon.com and buy half-price shopping vouchers good for everything from jeans to trendy messenger bags. And quite a few did; at one point more than 1,200 people a minute were snaring the coupons, so many that the site that arranged the “flash” sale had to redirect some shoppers and tell them to come back later. But by day’s end, almost 450,000 potential customers had bought into the deal. It’s the new paradox for the American shopper: Just when buying online has become second nature for most consumers, retailers are shaking up the way they sell, now increasingly using the Internet to lure them back to Main Street or the mall. Electronic coupons, long a ho-hum corner of the e-commerce world, have acquired a whole new buzz, with more than 350 flash-sale Web sites springing up in the past two years, some of them generating monthly visitor traffic in the millions. And mobile phone shopping apps, once niche toys that used GPS to help the tech-obsessed find the nearest discounted latte, are moving into the shopping mainstream too. With the make-or-break holiday season approaching, big-box retailers are ramping up their technology so they can beam product offers directly to shoppers’ phones—hoping to reach those buyers when they’re within a short walk or drive of the stores. About 33 percent of retailers plan to have “location-based marketing” technology in place by year’s end, up from almost nothing a year ago, according to the National Retail Federation. Analysts describe this electronic push as the latest move in a psychological chess match, as retailers try to keep consumer budget-consciousness from casting a pall over the lucrative end-of-the-year shopping season. E-commerce has proved resilient, despite the recent recession; tech analysts at Forrester Research expect it to rise 11 percent this year, to $173 billion. But analysts point out that shoppers still make far more purchases in stores than online, and a well-targeted coupon can entice people out from behind their keyboards. At the same time, phones with the software and GPS technology to accommodate “geo-targeted” sales pitches are close to becoming ubiquitous: One in five online consumers now uses either an iPhone or another smartphone. Mobile phone–based deals, which target these folks while they are shopping and even right in the store, could be something of a perfect weapon. Of course, much of the new technology has practical and even ethical kinks to work out. The tools give retailers more ways to track users’ locations and habits, bringing up privacy concerns among consumer advocates. And some consumers are already reporting getting inundated by local retailers’ offers. (Joel Osman, who heads a mobile-technology group for consulting firm Accenture, calls this emerging hassle geo­spam.) To help readers filter out that kind of hype, we’ve taken a close look at three of the fastest-evolving shopping tools. Resource: http://www.smartmoney.com/