1: Online Source of Shortwave Broadcasts – Clean, easy interface to find listings by the hour
2: SWL Archive – Excellent source of current info on worldwide SWL stations and an archive of SWL broadcasts for several stations.
3: Your Guide to Shortwave Radio Listening, News and Reviews – A beginner’s guide
4: Wikipedia – Concise overview of Spy Numbers Stations. Read this to better understand the history of Spy Numbers. A numbers station is a category of shortwave radio stations characterized by broadcasts of formatted numbers, addressed to intelligence officers operating in foreign countries.
5: Wikipedia – Shortwave radio entry briefly touches on many aspects of shortwave listening.
6: SWL Nostalgia Site – Loaded with interesting articles and links.