MuseScore

Screen dump: 
MuseScores main window

MuseScore is a powerful music notation editor for the creation, editing and printing of various types of musical scores.

Some features include WYSIWYG creation and editing, support for unlimited staves, unlimited score length, up to four independent voices per staff, percussion notation, a plug-in architecture, and excellent looking notation.

Full list of features

There is also a community site for sharing sheet music made with MuseScore -

MuseScore.com