Project Gutenberg – keeps getting better 2 months ago
If you like Project Gutenberg, the closest analog for music is IMSLP, the Petrucci Music Library (imslp.org) — over 855,000 public-domain scores maintained by volunteers, with the same labor-of-love energy and the same perpetual scan-quality and copyright-jurisdiction headaches. Same ethos of "the works belong to humanity, not a storefront." Worth a bookmark for the musicians on HN.