A growing number of online companies are ambushing competitors through software that puts ads where marketers want them most--in front of customers visiting rival Web sites.

It's called getting "Gatored," after one of the most popular applications underlying it, and some Web sites are out to restrict the practice. But for others it's fast becoming an effective way to feast on competitors in their own front yard. One e-tailer that's been bitten is 1-800-Flowers.com. When certain Web surfers visit the site to browse for bouquets, a pop-up ad appears for $10 off at chief rival FTD.com. The same sort of thing happens at AmericanAirlines.com, where a Delta Air Lines promotion is waiting in the wings. Ads like these find their way onto browser windows through "plug-ins" that come bundled with certain software downloads. Several companies are behind the practice, including the eponymous Gator.com. One online executive referred to Gator and other such programs as "hijackware," applications that easily whisk consumers from a point of sale at one site to a competitor’s site. Gator is a so-called online helper application that has millions of active users and manages passwords and user IDs. While Gator is free, the company that makes it sells keywords to marketers that lets them launch pop-ups at opportune moments--for example, when a Web surfer visits a competing site. Redwood City, Calif.-based Gator is quickly adding a new spin to the old ploy of selling ad space linked to trademarks and company names. Pioneered by search engines and Net directories, the practice lost momentum after some big brand advertisers cracked down with boycotts and trademark lawsuits. One case involving Playboy Enterprises, scheduled for trial next month in Los Angeles, could help set guidelines over the legality of the practice. Fears about control of keywords have resurfaced recently, with critics pointing to blurring lines between content and advertising on the Web. In one extreme example, San Francisco-based eZula has been working with file-sharing networks Kaazaa and iMesh to superimpose links to marketers' sites over text on Web pages. But including links to preferred Web destinations could rally a cry of foul play. Microsoft last month pulled--at least temporarily--Smart Tags from Internet Explorer 6. The Extensible Markup Language (XML) feature let Web surfers use pull-down menus to obtain additional information about select content. A Smart Tag under CNET, for example, might have linked to Microsoft's MSN MoneyCentral Web site for stock quotes and other information.