Podcasts were originally audio, many are now video (some are called webcasts). Many series offer subscriptions and most sites offer the choice of listening immediately or downloading for your own future use from your computer or portable player (e.g. iPod, tablet).
Tips and indexes
Assorted podcasts or build your own
Extensive, indexed in 16 categories
Podcasts created by Library of Congress
Science Talk and 60-second Science in podcasts
Center for Disease Control podcasts