Well, there is Dark Matters which is done by some guy in Australia.
There is Explorations with Dr. Michio Kaku every Wednesday.
There is Bad Astronomy Q&A
uhm... 60 Second Science (updated every day)
Nova Science Now
Nasa's Jet Propultion Lab (most are video, but have some audio)
and Science Talk - The Podcast of Scientific American

I'm sure there tons more, but those are the ones I get right now. (Along with Coast to Coast). If you have i-Tunes, you can just go to the i-store and search under science and find whatever you want.

ALL of the ones I meantioned above (except Coast to Coast) are FREE.
You don't have to pay to use i-tunes. You only pay for premium services and podcasts.