Sheet Music Notation Software

LilyPond is a music engraving program. From easy to type text input you can create professional PDF sheet music for free.
http://lilypond.org/

The Mutopia Project offers sheet music editions of classical music for free download (more than 1600).
http://www.mutopiaproject.org/

scorio is a platform where you can easily write, find and share sheet music … more
scorio is built on open technologies , that function with any Web browser … more
scorio has a free offer for you
http://www.scorio.com/

Thanks to Audrey for the tip!