According to Wikipedia, “A music library contains music-related materials for patron use. Collections may also include non-print materials, such as digitized music scores or audio recordings. Use of such materials may be limited to specific patron groups, especially in private academic institutions. Music library print collections include dictionaries and encyclopedias, indexes and directories, printed music, music serials, bibliographies, and other music literature.” (Music library, retrieved 17:21, 9 September 2011 (CEST))
Note: This page never will become an index of download sites and commercial sheet music. It's main purpose is to describe what digital music libraries are and to provide links open and free resources.
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Some of these MIDI files are not suitable for displaying the score in a digital piano as is. Loading it into a music notation software or some other MIDI edit tool and then saving may help ...
... there are many more
Consult the MIDI File Search Engines list at notation software.
MusicXML is a transport format for music notation that is useful in addition to MIDI since it can include much more information.
See the MusicXML Sites list at Recordare for more ...
Music scores can be published in PDF or other printable formats, as well as some kind of source format. Sometimes, MIDI files or audio files with free performances also can be found. Most free music is "classical" music, for obvious copyright reasons. Unfortunatley, not many modern artists agree to free their creations ...
See also: Other sheet music sites (at WIMA)