Podcasting is taking a step into the mainstream. Apple Computer Inc. yesterday introduced a version of its popular iTunes music software that includes a directory making it easier to find and listen to podcasts -- radio shows and other audio programs posted on the Internet for anyone to download. Many podcasts are produced by individuals, on a range of quirky subjects, but it hasn't always been easy finding the tens of thousands of available programs and downloading them to digital-audio players. The new directory -- a sort of TV Guide for Internet audio programs -- also makes it easier to...