NotesDeMusique is probably one of the best free and subscription-free apps to learn how to read music notes.

NotesDeMusique is a free, subscription-free educational musical game focused on the musical staff.
NotesDeMusique allows you to learn to read music notes on a staff while having fun, develop your ear through musical dictations, and offers many additional features.

♪♫ Features
✓ 4 types :
―― Reading music (notes)
―― Ear training (notes)
―― Reading music (chords)
―― Ear training (chords)
✓ 4 modes :
―― Training
―― Timed game (finding the maximum score in a 1 or 2 minutes game)
―― Survival mode (Game over if you make a mistake)
―― Challenge mode (Challenge on 5, 10, 20, 50 et 100 notes !)
✓ 3 notation systems to display the notes name :
―― Do Ré Mi Fa Sol La Si
―― C D E F G A B
―― C D E F G A H
✓ 4 clefs, on 4 octaves ! :
―― Treble Clef
―― Bass Clef
―― Alto Clef
―― Tenor Clef
✓ Save scores by types and games modes

♪♫ Additional features
✓ Tuner
✓ Dashboard and streak tracking
✓ Dictionary of chords
― Available chords are :
―― Major
―― Minor
―― 7 (dom)
―― 7 Major
―― 7 Minor
―― Dim
―― Aug
✓ Help displaying the notes name

♪♫ Contact
If you find any bug or if you have any suggestions to help improve NotesDeMusique, please contact me !

♪♫ Website
NotesDeMusique website: https://www.notes-de-musique.com
NotesDeMusique changelog: https://www.progmatique.fr/freewares/freeware-9-NotesDeMusique.html

♪♫ Discover the NDM Suite
NDM (Notes De Musique) offers a series of applications to learn different instruments:
― Notes De Musique : The first application focuses on reading music notes from sheet music (on the staff, for music theory).
― NDM - Guitare 🎸
― NDM - Basse 🎸
― NDM - Ukulélé 🎸
― NDM - Piano 🎹
― NDM - Violon 🎻