For those of you that didn't know, a "Podcast" is halfway between a blog and a radio program. In fact, most podcasters also maintain blogs that show similar content. They are easy to record and upload, where a special RSS aggregator then downloads the recording to your computer, iPod, or any device that can play MP3 files. (It's just called a PODcast because iPods dominate the market and it's a derivative of "BROADcast.")
Podcasts have two things going for them. First, because they're saved as mp3 files you can listen to them when they're most convenient. Don't time to finish listening to a podcast? Just hit pause and go back to it later. It'll wait for you.
Second, podcasts are narrowcasts. Broadcasts are usually done by companies that need to make money (even public radio needs to attract enough listeners to do well on a membership drive), so they will try to appeal to as wide an audience as possible. Ever wonder why there's so many conservative talk show hosts? It's because they have the most listeners so they get the most advertisers.
On the other side of the coin, podcasts are usually done just for the fun of it - like blogs. Sure, a few podcasters might ask for some donations, but since they're self funded they don't need to have a broad audience. As a result, they can focus on very specific topics that might not attract more than 1,000 people. (This does not mean podcasters try to avoid listeners, only that they don't have to cater to the masses) I personally listen to podcasts on web design, technology, photography, and educational technology. Some of those topics would never make it into a radio program, but they're important to me so I listen.
There are many good podcasters out there, but here are three good ones that teachers might want to check out:
This frequently updated blog includes many a podcast on how teachers can integrate technology into their curriculum. Far from being merely a blog about blogging, any type of technology is fair game to this guy. Next to Leo Laporte, he's one of my favorites.
The maintainer of this site is a Supervisor of Instructional Technology and Communications, so you can be certain he knows what he's talking about. Mr. Richardson describes himself as a "blogvangelist," so it's quite understandable that most of his postings are centered around how blogs can be used in the school setting. [EDIT - while Weblogg-ed is still quite active, he doesn't podcast anymore.]
Another good site, this podcaster focusses mainly on websites that can help teachers in the various subjects. It's important to note that this is not Teachnology.com - that's a different site and it's nowhere near as informative.