Beyond the pieces on the site, any standard MIDI file (.mid or .midi) plays as falling notes. How easy it is to practise from, though, depends a great deal on how the file was made. This guide goes by what the player actually does with a file, to explain what makes one good to practise with.
Finding files you are free to use
The pieces on this site come from the Mutopia Project, which publishes engravings and MIDI files under public-domain and Creative Commons licences. Every song page links to the original with "View score on Mutopia Project", and the project has many pieces that are not collected here.
Exporting MIDI from notation you have entered yourself is another dependable source. For a piece still under copyright, or someone else's arrangement, check the licence it was published under. Whatever you open is read inside your browser and never sent to the server.
Opening a file
Press Load MIDI File on the home page, or drop the file on the box marked "Select or drag & drop a MIDI file" on the MIDI Player page. Once it loads, the duration, the number of notes and the number of tracks appear under the title; the track count is worth a glance, for the colour reasons below.
When playback starts there is one bar of silence before the first note. The player adds it, to give you time to get your hands in place. A file you open yourself has no sheet music view and does not run the practice timer; to log the time, use the practice stopwatch or Add a record on the Practice page.
What the player does with your file
Each track gets its own colour, from a set of eight used in turn. Instrument assignments are ignored and every track plays on the piano - a drum track too, which then falls onto the keyboard as piano notes - so remove anything that is not piano before exporting. Notes outside the 88 keys are not shown.
Velocity drives both the loudness and the brightness of each bar. The damper pedal (CC64) and soft pedal (CC67) appear in the pedal display and affect the sound. Channel volume (CC7) and expression (CC11) are applied per channel, so a balance between the hands set in the file is played as written.
What makes a file good to practise with
Most important: the right and left hands on separate tracks. In notation software, entering the upper and lower staves of the grand staff as two staves usually exports them as two tracks, and the hands then fall in different colours - which is what makes hands-separate practice easy.
Beyond that: set the instrument to piano; let dynamics come through as varied velocities; enter pedal marks if you want the pedal shown; set the opening tempo correctly, since the BPM on screen is worked out from it; and leave no extra silence before the music starts.
Common problems and fixes
If every note is the same colour, both hands are on one track: split them onto separate staves in your notation software and export again. If the colours are scattered across many tracks, the file splits by voice; regrouping the voices by hand makes it far easier to read.
If one part is very quiet or silent, the file sets that channel's volume (CC7 or CC11) low; check the volume settings in your software. If notes ring on and turn to mush, a pedal-down was probably never released - see whether the Sustain gauge stays full.
If opening a file from Load MIDI in the header, or from a song page, shows "Failed to save the file. It may be too large.", the file is too big to be handed from one page to the next; open it on the MIDI Player page instead. MIDI files of piano pieces are normally small, so also look for unneeded tracks or a mass of controller data inside it.