Dennis Harmath has built a LilyPond IDE for Eclipse. He sent a very nice testimonial to our newsgroup:
I'm happy to announce Elysium, a LilyPond IDE for Eclipse - proudly brewed with Xtext!I must say that I really enjoyed developing tooling support for LilyPond, the definitive textual DSL of music notation using Xtext, it was great fun to see the grammar evolve with the help of automatic abstract syntax inference, and the IDE features I got "for free" from Xtext really impressed me. Hooking into Xtext's build process to implement incremental build was also much easier than writing an own builder. In the future, developers might reuse LilyPond's abstract syntax as a metamodel for music notation e.g. to write converters from/to LilyPond, hence bringing more music notation support to Eclipse.
There's also a screenast showing Elysium in action: