LilyPond is a music typesetting system and markup language. The internal parts are written in lisp and the user interface is influenced by the TeX syntax. So, are questions about this system on-topic on any of StackExchange sites? And on which one?
Typical question:
How do I add a tie on seconda volta note in LilyPond?
LilyPond seems to have a small following at Musical Practice & Performance
I think you'd be surprised at just how many musicians use utilities like this to reproduce music.
There is also a few at SuperUser.
I think either site is fine. I'd start with Musical Practice & Performance, as it is much more specific to that crowd.
There's an argument to be made that - as they effectively contain code - LilyPond input files can become so inscrutable to anyone but programmers, that questions about them are (a) unlikely to find answers on Music.SE (or, to a lesser extent, SuperUser), and (b) on-topic as programming questions for Stack Overflow (though this is maybe a bit more controversial).
On the one hand, this is similar in principle to asking how to use a particularly complex GUI to achieve the same result, making the question purely about usage and so off-topic for SO.
On the other, you could argue that a non-programmer on Music.SE is much more likely to be using the GUI than LilyPond anyway, and - depending on the complexity of the required solution - not equipped to actually formulate an answer by writing code. It doesn't seem sensible to ask questions in "the right place", when they stand a better chance of being answered in "the slightly less right place".
Of course, we could just conclude that they're off-topic everywhere on SE at the moment, and so should be confined to the LilyPond mailing list, but that sounds overly dogmatic and inflexible to me.
It's been about 10 years since this question was asked, so I thought I'd provide an up-to-date answer.
Currently, there are 3 sites with a LilyPond tag:
Music: Practice & Theory is without a doubt is the best SE site to go to for general questions on LilyPond.
It has a [lilypond] tag which hosts more than 70% of all SE's LilyPond questions. This is over 600 questions and over 900 answers (with less than 2% of questions going without upvoted or accepted answers).
It also has over 40 questions with the [lily-scheme] tag. This is for questions about Scheme functions within a LilyPond context. Scheme (from the Lisp family) is one of the lower level programming languages that LilyPond is built with.
As a side note, I was the first user there with a shiny silver tag badge for that tag. Having said that, there are certainly many other SE users that I know of who have more experience and expertise.
Stack Overflow also has a [lilypond] tag with a considerable number (about 20%) of the overall questions. But they don't seem to get nearly as much attention.
TeX – LaTeX is also worth considering. While it has less than 10% of the total questions tagged with [lilypond], most of these are about integrating LilyPond within TeX documents.
There are also a handful or so questions on Super User that at least mention LilyPond.
Expanding on Frankston's answer (i.e. commentary that is too long to format as a comment):
I've always considered lilypond to be directly on topic for Musical Practice & Performance, given the indication that "usage of specific music software" is on topic. I'm certain that there are many users who are experience in how to read lay-out music notation in general, and there are a few who are using lilypond specifically.
I can see an argument that more complicated lilypond scripting, i.e. once users start to program in guile within their lilypond files, could be more of a Superuser or Stack Overflow type of question.
However, to date it looks like all of the questions are "how do I get lilypond to output what I want" (Music Practice) rather than "how do I do complicated programming within a lilypond file" (SuperUser), so I'd still say that the primary location for basic lilypond usage questions is Musical Practice & Performance.
As a final note, prompted by PeterBjuhr comments/answer, one of the reasons why there are so few questions is because lilypond has an active and helpful mailing list; and you're likely to get better answers there (at least for now).
It has been recommended by the LilyPond community that all questions should be posted to the LilyPond Users mailing-list. There you will get fast and competent help!
For details how to use the mailing list, go here.
For a more thorough searching of old threads you can use: http://lists.gnu.org/archive/html/lilypond-user/
If you like to get more involved you can also subscribe to the list.
