The Secret Broadcast is a numbers station podcast. Every week a new episode is published. The content of the podcast is encrypted messages. You can submit a message and it will be read as an encrypted sequence of letters and numbers, and which can be decrypted with a key only available to you and anyone you share it with.

An archive of pirate radio broadcasts from the late 1990's at archive.org. You can download them for free or listen to them online. The quality of some of the recordings is kind of poor (it is pirate radio, after all) but occasionally you'll come across a gem in the rough.