I too contribute monthly to Petrucci. Apart from downloading the scores of piano pieces I want to play (they offer a wide set of arrangements for all but the most obscure pieces), I have discovered several hidden gem in their library, which grows more and more every time I check!

Before Petrucci, I used to be a regular user/contributor of the Mutopia project [1]. Its purpose is to provide Lilypond [2] source files for scores: this allows users to create MIDI files or re-create PDF files using custom page layouts/line breaks/etc. These features are handy, but creating a Lilypond file from a score is much more time-consuming than simply scanning it and uploading to Petrucci. (Each of Mozart's and Haydn's string quartet I uploaded took me ~2 weeks of work.)

[1]: https://www.mutopiaproject.org/

[2]: https://lilypond.org/

i don't work for any of these sites but i wanted to say thank you, just as a human