"Keep Circulating The Tapes" was a phrase often seen during the earlier seasons of MST3k due to the show's cult reputation. Many viewers continued to spread word of the show's existence by sharing it with their friends and relatives by taping the show on a regular basis.
A phenomenon that happened early in the shows life was tape trading. Even while it was still on KTMA viewers of the show started taping it to show to their friends. This was a major impact on getting people to tune in to its odd time spot. During the first few seasons of the Comedy Central years a tribute was given to these early promoters with the credit reading "Keep Circulating the Tapes."
Many websites are set up to allow trading of tapes and DVDs. There is even a section of the Satellite News site that was once devoted to fan trading.
Selling, and copying to distribute copyrighted material is prohibited. However, direct trading of tapes and DVDs is somewhat unregulated.