For users based in Germany or Austria or German-speaking users only:

Flugfunktrainer
Practical and theoretical exam preparation for German aeronautical radio operator certificates

Flugfunk AT
Aeronautical radio simulation at Austrian international airports and additional radio training features

FunkTraining2Go
Exam preparation for German ham radio licenses as well as maritime and aeronautical radio operator certificates

Schiffsfunk
Interactive exam preparation for German maritime VHF radio operator certificates (theoretically and practically)