Free and Open Notation

MuseScore is a promising free WYSIWYG music notation program, licenced under GNU GPL. It already has a set of features superior to Finale Notepad or other entry-level notation packages (offering, for example, unlimited staves and a set of microtonal accidentals), making it ideal for most classroom applications and probably good enough for serious projects requiring only conventional notation.  It appears that the programmers are actively working on some additions required for more complex scores like nested tuplets.  It's cross platform, but currently only for Linux and Windows, with Mac available only in a prototype version.  The output I've seen from MuseScore is comparable in style to that of Lilypond.