Ear training

Artusi - A huge set of free online courses on ear training (melodic, harmonic, rhythmic dictation), four part-writing and counterpoint where you type notes directly in the browser. (Does it make Counterpointer obsolete?)

Miranda Wilson. Integrated Musicianship: Aural Skills - An open-source, interactive, online textbook for college-level music courses
SoundGym - Ear training for mixing engineers: EQ, compression, stereo image, gain, reverb
ToneGym - Intervals, chords, rhythms (with some gamification)
Perfect Pitch - Intervals, chords, scales, inversions
🇷🇺 Сольфа - Музыкальные диктанты. Советую также попробовать MacGAMUT Dictation на Artusi
https://www.teoria.com/en/exercises/
Auralia
https://www.freemusicdictations.net/

Sight Singing

(TODO: make a separate page maybe)