Ira Glass, creator of "This American Life." Evan Agostini/Invision/AP

More than 25% of internet users download or listen to podcasts, Pew reports. Apple reached its 1 billionth podcast subscription last year.

Podcasts are gaining in popularity for a number of reasons: You can listen in the car or on the train to work, you can dig deep into a topic, and you don't have to burn your eyes out on a screen.

To that end, here are nine podcasts that are sure to stimulate your brain.