I've got a travel-size violin (by Bill Whipple) that I mess around with. I like a lot of traditional and celtic music, and I wanted something that would provide both a visual and aural rendition of a song to help me learn it.
Looking around, I found ABC (see John Chamber's page and
tutorial), which is a simple, ASCII notation for sheet music.
There are lots of software tools for processing ABC; my favorite
is abcMIDI (which I've packaged in my Gentoo overlay and is
also packaged in the Sunrise overlay).
abcMIDI provides several
abc2midi: ABC to MIDI conversion, to listen to your music.
yaps: ABC to PostScript conversion, to look at your music.
midi2abc: MIDI to ABC conversion, when you can't find ABC source.
To make playing ABC files as easy as possible, I've written a little
script, abcplay.py, which uses
abc2midi to generate a MIDI file
for each tune and then plays them with timidity++. Because the
processing is so similar,
abcplay.py also plays LilyPond files,
lilypond to convert them to MIDI. Run
$ abcplay.py --help
for usage information.