Learning how to sing Gregorian Chant? If so you will love iChant Gregorian. Use it to learn the notes, teach it to others, remind yourself of the starting notes, and work with pianists, organists and other musicians. Tap the note to play the corresponding piano sound.

Easy as Do Re Mi.

Features:

-Specially focused on the four-line Gregorian staff.
-Transpose notes up and down.
-Absolute pitch of last note played indicated in lower left corner.
-Select Do or Fa Clef and line position.
-Note names displayed above notes.

Also available as an Android app:
https://market.android.com/details?id=com.bengeorge.ichant