Leveraging Ajax, the DOM and Lilypond for a web-based notation editor, from the latest edition of the Lilypond Report.
The article discusses OMET:
OMET, an acronym for Online Music Editing Tools, is a project to develop a group of web-based applications for creating professional-quality music manuscripts.
Basically, this is a browser-based GUI Lilypond editor. Very, very cool.
Right now it's limited to guitar scores – though nobody should take that to mean notating guitar music is a simple task, given all the resources of the instrument. Add to that the nature of polyphonic music notation on a single staff and, well, "issues" arise.

I signed up for this project's mailing list to keep up-to-date with its progress.
BTW: Is that Fernando Sor's Opus 6, #6 I see in the article's screenshot?
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Also check:
http://www.weblily.net
http://www.scorio.com
Thomas Bonte
26 Sep 10 at 11:42 am
Thanks Thomas,
I’ll be sure to check these out and post about them, too.
nitin
26 Sep 10 at 12:23 pm
I just launched LilyBin.com. The goal is very similar to the tools already mentioned, but I think I've made the tool easier to use and integrate into the daily workflow. No registration is required, and it integrates nicely with Dropbox.
Trevor
11 Feb 12 at 4:46 am
Cool Trevor! It’s looks great. Thanks for letting me know.
nitin
19 Feb 12 at 7:58 am