Podcasts are audio shows that you can download to your computer and optionally to an mp3 player such as an iPod.
The cool thing about these podcasts, is that you can subscribe to them, using software such as iPodder, and have new shows automatically sent to your computer. I'm using iPodderX for the Mac.
Adam Curry's Daily Source Code is my favorite.
I listen to it every day. I usually load it onto my iPod and then take a walk. Curry's show is usually about an hour long, so it encourages me to get out there and excersise every day. The cool thing about podscasting is that it's like TiVo for audio. You can choose the shows that you want to listen to and when.
Doug from Clint and Doug's Spectacular Craptacular says I should listen to RadioZoom.
I also liked listening to Coverville Episode 13: A Special All-Cover Tribute to ABBA today.
I found Doug's show on the Last 100 podcasts page.