Musopen is a 501(c)(3) non-profit focused on increasing access to music by creating free resources and educational materials. The site provides recordings, sheet music, and textbooks to the public for free, without copyright restrictions.
MusicBrainz is an open music encyclopedia that collects music metadata and makes it available to the public.
Public domain classical and folk music free to download.
Free to download sound effects.
Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more.