"Clair de Lune" is one of the most recognisable pieces of the piano repertoire (thanks to many television adverts) and was written by French composer Claude Debussy. Clair de Lune (or Moonlight) is the 3rd movement from his Suite Bergamasque for solo piano and is an atmospheric piece of music, in what is called the composer's "impressionist" style. You should play it rubato (with a gently impulsive rather than a strict tempo), and let the musical ideas flow and grow. The complete "Suite Bergamasque" for piano, consisting of four movements in total, is available in a number of editions including these from Sheet Music Plus and The Music Room. Below you can follow the music with this graphical animation, play the mp3 file and see the sheet music score before downloading.
Here is a video of Debussy's Clair de Lune for piano:
The image below is the first page of the sheet music, and here you can download the full sheet music for Debussy's Clair de Lune for piano in PDF format.