MuseScore is a free cross-platform WYSIWYG music notation program that offers a cost-effective alternative to commercial programs such as Sibelius, Finale and Capella. It is a free and open source music score creation application that's customizable, powerful, attractive, comprehensive and well-designed.
Here are some screenshots to illustrate the features and user interface of this application:
Synthesia
Virtual MIDI Piano Keyboard
Everyone Piano
Hydrogen
Guitar and Bass