What packages have you found in AUR but not in Debian or Fedora?

As others have pointed out, Debian and Fedora have very large repositories. More to the point, Debian and Fedora are the main entry points for software applications in the Linux ecosystem. Most other Linux distributions are based on either Debian or Fedora. Arch is probably the most popular exception.

I do have the impression that the barrier to entry to Arch's repositories is lower, so perhaps some new projects do make it to Arch first.

Occasionally, when there's some software I want that I can't install through Debian's package management, I'll go to the developers' site and follow their installation instructions.