Cyberduck (started in 2002 by David Kocher) gained popularity for its clean interface and broad protocol support. From 2005 to approximately 2015, users could request a registration key after making a donation. Without a key, the software displayed a brief donation reminder on launch but remained fully functional.
Several factors drive this search query: Cyberduck Registration Key
To ensure you have a clean, working installation without any fake keys: Cyberduck (started in 2002 by David Kocher) gained
If you ever see a window asking for a license key, you have downloaded a fraudulent or modified version. Delete it immediately. If you ever see a window asking for
David V. Kocher, the lead developer, has publicly stated multiple times on the Cyberduck mailing list and GitHub issues:
“Cyberduck does not require a license key. If you are asked for one, you are using a counterfeit version. Please download from the official website.”
He also notes that the project has never sold registration keys. The only paid distribution channel (Mac App Store) never used in-app keys.