Crop Circles Magazine is a publication mentioned in "Megadoomer", "The Nightmare Begins", and "Dark Harvest" devoted to the paranormal. It's a magazine that Dib is subscribed to, and he regularly mentions submitting proof of Zim's alien origin to it. Based on this assertion, it is apparent that Crop Circles regularly takes people's submitted proof of paranormal encounters, and publishes it in its pages.
A cover of the magazine has never been seen, and it is unknown whether the employees are aware of Dib's antics. Subscribers can also order things such as Alien Sleep Cuffs and X-Scope from the back of it which actually work; these ads are likely purchased advertising spaces rather than products directly affiliated with the magazine itself.