Also for layout algorithms. The people behind lilypond have thought very hard about what makes a score beautiful and legible and have pushed the art of computer engraving forward immeasurably. This is the TeX for music. Sibelius, Finale et al. have only recently begun to catch up.
Lilypond's web site could benefit from some examples of its output directly on the front page. I tried three different approaches in my casual attempt to see just how good it is, and didn't reach any PDFs or printouts.
Lilypond has a working syntax but it could sure do with some simplification and consistency. I use Lilypond, I use Latex, I know Lisp, but I find the Lilypond syntax can be mighty confusing.