ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
ChordPic - Create beautiful guitar chord diagrams
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.
Hyperlax - Sit back. Relax. Explore faraway places. Enjoy the music.
Finale - Finale, the world standard for music notation software, lets you compose, arrange, notate, and print engraver-quality sheet music.