NPR radio show This American Life is now available as a free podcast. I've been wanting to listen to it for a while and I'm really enjoying it now that I can hear it on my commute. Other podcasts on my iPod:

Battlestar Gallactica - Instant DVD commentary for the best sci-fi show on TV.
Meet the Press - Tim Russert refuses to take BS off of anyone.
News Hour - Nightly PBS news show with another great reporter, Jim Lehrer. These are broken up into segments, which is really nice. I listen to the stories I'm interested in, in the order I want.
Now with David Brancaccio - Weekly PBS news magazine that is unabashedly leftist.
Le Show - Harry Shearer of Simpsons fame. Comedy, news and commentary.
Onion Radio News & President's Weekly Radio Address - The Onion is funny in audio form, too. Their Bush speeches are the best, though.
This Week in Tech - Successor to The Screen Savers and the grandfather of all tech podcasts.

I'm not giving links because you can just look in iTunes or your favorite podcast directory if you want to subscribe. There are some other shows on my iPod, but these are the shows that I never miss. Having time to listen to all of this is one good thing about being in my car 8 hours a week.