Johann Sebastian Bach wrote the first piece in the huge work "The Well-tempered Clavier", as a Prelude in C major. This comparably easy piece is then matched with a (harder) Fugue in C major. The whole work takes you through all 24 major and minor tonalities with one prelude and one fugue of each.
The prelude in C major is beautiful as it is, but since it is composed with only broken chords, it is easy to understand anyone being "tempted"to add a melody to it! Gounod did so. This became the famous Ave Maria, a real hit!
Get Ave Maria for Easy Piano here.