Musicians love reading from great-looking scores, but programs that produce them (such as Finale and Sibelius) can be quite pricey. LilyPond is a free, albeit much more difficult-to-use, alternative that produces excellent-looking results. The difficulty stems from the lack of a WYSIWYG interface--LilyPond simply interprets text files, which you or others must provide.
LilyPond's text files aren't particularly difficult to understand or produce. However, the commands and syntax can also be as complex as music itself. Music entry is tedious, even with a mouse, and with LilyPond it can be downright drudgery. For instance,
title = "A scale in LilyPond"
c d e f g a b c
produces a C scale in the treble clef. That said, the results I produced in PDF form were nothing short of splendid--as good as or better than any program I've ever tried. Also, you can reap the 1500 fruits of other people's efforts by downloading LiliyPond files from Mutopia.
LilyPond is part of the GNU project and as such, represents a fair amount of altruism on the programming and music community's part. When some of the other music programs out there start exporting LilyPond files--or when someone sticks a visual editor on top of it (there is at least one at http://www.denemo.org/ in development)--it's going to be something else. As it stands, it's something anyone who wants to produce good-looking sheet music should at the very least familiarize themselves with.