Notation Pad - Notation Pad is the Easiest way to write sheet music with notations on iOS/Android.
NtEd - Music Score Notation editor.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Orpheus App - Sheet Music Reader for Android
DaCapo Editor - DaCapo Editor is a free Open Source music sheet editor which allows to easily create nice looking partitions. It is available as a web application as well as a desktop application for Windows, MacOS and Linux.
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.