Why don't you make one?
A "podcast" is a downloadable audio format you get off the internet, which can be played on an MP3 player (like the eponymous iPod) or on your computer.
Many radio stations, like NPR, now feature downloadable podcasts of their radio shows.
Additionally, many, many people have begun putting podcasts on the web, where they can discuss any number of subjects.
Predictably, they are big in the "geek community". For instance, the movie website chud.com often has podcasts available by the site's contributors, where they wax about various pop cultural events.