Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Openwhyd - Collect and Share the tracks you love.
MuseScore.org - Create, play back and print beautiful sheet music with free and easy to use music notation software MuseScore. For Windows, Mac and Linux.
Music Smasher - Find any song. Music Smasher searches Spotify, Rdio, SoundCloud, Bandcamp, and YouTube.
Finale - Finale, the world standard for music notation software, lets you compose, arrange, notate, and print engraver-quality sheet music.
Zoff - Collaborate on a live music and video playlist with your friends.