Podcasts -- those amateur or professional audio or video programs delivered automatically to a subscriber's computer or MP3 player -- let consumers listen to their favorite shows whenever and wherever they want. But though the technology for podcast subscriptions has been around for several years, the mainstream has only recently caught on.

An explosion in podcasts' popularity in the first half of this year, culminating in the launch of a podcast directory at Apple's iTunes online music service, has providers scrambling to keep up with server demands and businesses looking for ways to turn a profit.