Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
The iTunes Store is host to thousands upon thousands of podcasts, those free audio (and video!) programs put out by everyone from big television networks to a guy in his basement with a microphone.
If you want to see what podcasts are available, click the Podcasts link on the Store's main page. You're then whisked to the Podcasts section, where you can browse shows by category, search for podcast names by keyword (use the Search iTunes Store box), or click around until you find something that sounds good.