NPR and its podcast directory are great but somehow I have always liked BBC a little bit better. BBC 4 has a streaming Listen Again page and recently launched a comprehensive podcast directory. Subscribing is YouTube-style easy -- you can copy the URL directly, or subscribe via Google Reader/iGoogle or iTunes. Below is the list of some podcasts you may find interesting:
Best of Natural History Radio
Best of Today
Radio 4 Choice
Image source: Medical Matters, BBC.