The market for food and fitness tracking websites and mobile applications on the market has exploded in recent years, with many products tapping into the hallmarks on which weight-loss powerhouses like Weight Watchers and Jenny Craig have built their empires: daily tracking and accountability. A review published last year of 204 applications that were on the market in 2009 found that few applications did much more than offer a medium for keeping a food and activity journal or tracking weigh-ins. But weight-loss and technology experts say the potential for mobile products in weight loss is huge, and the market is changing quickly.