After upending the print media world, Yahoo! ( YHOO) has turned its attention to broadcasting, adding Web-based audio programs known as podcasts to its lineup of services. The podcasts available at podcasts.yahoo.com and next.yahoo.com range from National Public Radio shows to a program for knitting enthusiasts. Yahoo!'s offering, which is still being tested, will also provide lists of the most popular programs and allow listeners to rate their favorite shows. Listeners can hear podcasts for free. Yahoo! continually adds new features and content to its site, already the most popular on the Internet, to attract online advertising dollars away from rivals such as Google ( GOOG). Last week, Yahoo! acquired event planning site Upcoming.Org, and it has recently added new content including columnists for Yahoo! Finance and reports from war correspondent Kevin Sites. "As Web users increasingly become both consumers and publishers of content, Yahoo! continues to roll out innovative services that bring the power of Yahoo! technology to users," said Geoff Ralston, chief product officer of Yahoo!, in a statement. Podcasts played over Apple ( AAPL) iPods are growing in popularity. The Diffusion Group, a market researcher, estimates that 56.8 million consumers will be listening to podcasts by 2010, a more than tenfold jump from 4.5 million this year. Media outlets including The Wall Street Journal and the online magazine Slate offer podcasts, as do radio personalities such as Rush Limbaugh and Sean Hannity. "Clearly in a competitive marketplace, podcasting is becoming another form of distribution that you have to have to be competitive," said Diffusion Group analyst Marc Freedman in an interview. Clear Channel Communications ( CCU) has 38 radio stations with podcasts. The podcast audience is growing at about 10% per week, a spokeswoman said. Clear Channel is the largest owner of radio stations. On Monday, Yahoo! was at $34.16.