Full courses (free, no catch)

• Piano Video Lessons: the most course-like free option. Year 1 is Units 1-6. All videos are free with no account or credit card (also on YouTube). Unit 1 has 16 lessons: 8 songs, finger exercises, sight reading and the grand staff. Printable workbooks cost extra.
• Hoffman Academy: 400+ free video lessons in a unit-by-unit path, from keyboard basics up to Für Elise. Aimed at kids 5 and up, but the adult page uses the same lessons. The paid tier only adds practice plans, games, play-along tracks and sheet music.
• Zebra Keys: 50+ text lessons in four tiers (Preparation, Beginner, Intermediate, Advanced) with theory, chords and improvisation. No registration. Some pages are old.

Free samplers (email signup, then a paywall)

• Pianote: free 4-lesson "Getting Started" course, no payment method needed. Also a 5-day course and a blues bootcamp. Collects your email and sends marketing.
• Piano Marvel: free tier limited to the first lesson level and 3 sight-reading tests. Needs a Windows or Mac computer and a MIDI keyboard.
• Pianoforall: 7-day trial, then a $49 one-time payment. Asks for payment details up front, so it is a trial rather than a free course.
• Piano With Jonny: lots of free lessons, but mostly intermediate or jazz. The beginner learning tracks are behind a 14-day trial.

Theory and ear training

• musictheory.net: free with no ads. Theory lessons plus ear-training exercises (notes, intervals, scales, chords).
• Edinburgh "Fundamentals of Music Theory" (Coursera): 6 modules. Audit availability varies, so check at enrollment.
• BerkleeX "Introduction to Music Theory" (edX): 6 weeks. Marked partially free; the certificate is paid.

Reading sheet music

The full courses above (Piano Video Lessons and Hoffman Academy) teach the staff and clefs as they go. These are for learning to read notes quickly. Start with a lesson site, then drill with a game or trainer.

Lessons

• Sight-reading-practice.app: 19 free lessons on reading music, each ending with a short exercise.
• teoria.com: Reading Musical Notes: the staff, the treble clef and the line mnemonic, with a clef-reading exercise to follow.

Practice drills

• MuseScore Note Reading Trainer: free trainer for treble, alto and bass clef. Starts on treble and lets you switch clefs.
• Note Reading Practice: free, no sign-up. Treble, bass and alto clef.
• Notebusters Note Dash: free game for treble and bass clef that gets harder as you level up.
• NoteTeacher: 7 difficulty levels and an on-screen piano. No ads or registration. A MIDI keyboard is optional.
• Sight Reading Factory: generates fresh exercises. The free account is capped at 20 exercises (the pricing pages are old, so confirm).

Learn a specific song

Type a song name and see which keys to press. No single site does everything, so the usual combination is a letter-notes or falling-notes site for the melody and a chord site for the left hand.

Melody: which keys to press

• Easy Piano Letters: letter notes, and the keys light up during playback. Every song is free with no account, per the site.
• Online Pianist: falling keys, finger positions and letter notes. A few songs are free; an account unlocks more.
• Super Simple Piano: falling notes plus a lead sheet with chords. Free with 30-second previews; the first ten songs play in full, then it is paid.
• Virtual Piano: free on-screen keyboard that highlights the keys to press. You can play it from your computer keyboard. No account.
• La Touche Musicale: letter-note tutorials with video of the notes descending, and free sheet-music PDFs.

Chords and sheet music

• Hooktheory TheoryTab: free melody and chords as a color-coded piano roll, with MIDI download and a beginner filter (songs using only I, IV, V and vi chords).
• Chordify: chords for any song or pasted YouTube link, with piano diagrams. Playing along is free; transposing and printing are premium.
• MuseScore: a huge sheet-music library with playback. Keyboard highlighting, transposing and export are PRO, and some reviews complain about trial billing, so read the terms.

Beyond beginner

• PianoPig on YouTube covers beginner through advanced, per several roundups.
• Josh Wright's free tutorial pack for classical technique, across four levels (a few years old).
• Free sheet music: IMSLP and 8notes.

Suggested path

Start with Piano Video Lessons or Hoffman Academy as the main course. Add musictheory.net for theory and ear training, and Zebra Keys for chords. Free options give no feedback on your technique, so a few paid lessons with a teacher help later.