Also worth a look is Lilypond http://www.lilypond.org/.

Just a sidenote that the Opusmodus software is not open source, not free, and works only on Mac.

What information does OMN contained that isn't preserved in sheet music already? I was looking at OCR tools a few months back and found OpenOMR which expresses the MIDI instruction with all of these properties.

Looking at their score example at the bottom of the page, the right-hand score is defined as a separate entity from the left-hand score. As a result, to the OMN notation reader, it's hard to see the inherent synchrony between the two hands. This would be a bitch to debug and compose (i.e., code) without its display on a standard staff, which would show the time-ordered parallelism directly.

There's a conceptual issue with the language and a missing example: what of chords where one part of the chord is pressed longer than another? Or when one part must be staccato and another not?

I can't seem to see how this is expressed with OMN.

It would be easy enough to write a parser for this.

Interesting that they see it as a way to freeform new compositional ideas.

Over the years I've tried a huge selection of music composition software. I've yet to find something that's both easy and composable. It may be, just like the CLI/GUI discussion -- plain text may win out over a lot of mousing around and clicking.

Very cool! Looking forward to experiment with it. In the same spirit but less advanced (my "language" has bugs, I know) I wrote this toy project: http://sound-of-ascii.herokuapp.com/. There's a demo and you can play around with it.

Does this have notation for quarter tones? I can't find that on their website.

That's always been a pain point for me. No quarter tone notation rules out a lot, including most of Arabic and Turkish, and much of Greek maqam music.