Lilypond is probably the best free music notator and can face commercial monsters like Finale and Sibelius.
Lilypond is a code-driven editor and can generate PDF, PostScript, SVG, PNG and Midi file simply writing the code in a text editor. If you’re not code-minded, anyway you can use some editors and GUI for Lilypond
After a one-year work, Lilypond staff has released the 2.12 ‘Rune’ version, dedicated to Rune Zedeler (staff member, musician and contributor for six years recenltly passed away)
New features and bug fix list is here.
Lilypond is not easy …

One of the best music notation software, good for drums and drumline scores writing, too: Lilypond, with is ascii interface, is definitely a low-carbon software. And it’s free. And it’s complicated, and fascinating. A real good text editor, with pdf preview, tools for macro and template writing, simple parser and other good features is LilypondTool. Gui for Lilypond on posix sistems are Rosegarden and Canorus (also available for Windows).
If you don’t like command-line stuff try Finale Notepad. It’s for free too