I have these in my RSS reader, where I let them mass up until there are hundreds. Then I go through and download all the ones that sound interesting to a folder and burn them to a CD (as MP3s) or store on a thumb drive. These are listed in order of my personal preference:

• Science Friday is indispensable. I’ll download hours worth of shows for company on long, thoughtful drives.
• Dr. Kiki co-hosts The Week in Science, another great science podcast.
• Quirks and Quarks is a longtime (by interweb standards) standard.
• Groks Science Radio Show (formerly Berkeley Groks) is fun for it’s rough, yet erudite content.
• Scientific American’s 60-Second Science makes for nice, quick blips of Science Data.
• The Microbworld Radio Show is another 60-Second podcast. I’ve downloaded the archives for regular listening.
• From the prestigious AAAS comes a weekly review of the most recent published research.

With MP3’s, daily links, science videos, and books, there’s not reason not to have science everywhere you go!