If you're stuck with Chad Pennington on your roster, you may have had it with fantasy football. But for Yahoo! ( YHOO) and its rivals, the game is just getting started. As deep-pocketed giants like Yahoo!, Google ( GOOG) and Microsoft ( MSFT) jockey for Internet dominance, sports fans are emerging as a key battleground. The big Web companies are looking to grab loyal customers who will draw big advertisers. Emerging as a solid traffic driver: fantasy football leagues, where friends face off by stocking teams with real-life players -- such as Pennington, the injury-riddled New York Jets quarterback -- and comparing their statistics. The Yahoo! Fantasy Sports Web site attracted 5.6 million unique visitors in August, according to comScore Media Metrix. That was about double the 2.9 million registered by Disney's ( DIS) ESPN.Com, the next largest service, comScore said. The market research firm estimates that there were 10.7 million unique visitors that month to all fantasy sites. The figure measures people who view content such as articles about player injuries. Football is by far the most popular of the fantasy sports. Some 6.5 million people will play in Yahoo!'s fantasy leagues this year, up more than 10% from last year, says Nathan Noy, founder of Drafthelp.com, a Web site for fantasy sports hobbyists. This season, News Corp.'s ( NWS) Fox and Viacom's ( VIA.B) CBS are adding fantasy football coverage. Yahoo!, which began offering fantasy football in 1999, declined to comment on either Noy's analysis or comScore's figures. But it's clear that the company attracts users by letting them play for free. Players can add services including real-time statistics for $24.99 per team for a season. But mostly, Yahoo! makes money on football by selling advertising. "They like it because if you are going to be camped out there a little bit longer on the site, the chance of you doing something else like click on an ad increases," says Martin Pyykkonen, an analyst with Hoefer & Arnett who rates Yahoo! outperform.