Notation Pad - Notation Pad is the Easiest way to write sheet music with notations on iOS/Android.
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.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
StaffPad - Write music naturally on your Windows Surface Tablet.
PreSonus Notion - Professional music scoring software for desktop and iOS mobile devices.
Orpheus App - Sheet Music Reader for Android