Literally ANY topic which interests you that can be found through the "search" feature. Podcasts range from about 20 minutes to well over an hour, depending on if there are interviews included and how much information is broadcast on the given subject. Podcasts are generally updated frequently...some on a daily basis, and some are updated very infrequently. Your MP3 player will catch them all, according to how you set up your preferences. Or, you can download, save and listen to a podcast directly from your computer.
Here are my few purchased audiobooks recently acquired from iTunes, (not including the free loaned books from the local library):
We won't even get into the applications which can be download free or for a minimal fee. Check it out. There is a world of knowledge to be gained through podcasting.