The Blue Danube is probably the most famous waltz composed by Johann Strauss II (the younger Johann, son of Johann senior). You may have heard "The Blue Danube" many times on television of in films, and the waltz was used for example in the film 2001: A Space Odyssey to accompany a sequence involving spaceships. This version is a simple arrangement of the main theme for solo piano - download this simple arrangement in the following formats as PDF Sheet Music, as a MIDI file or as an MP3 file. This theme is also available on mfiles as an arrangement for Violin or Piano with alternative parts for either clarinet or horn and piano.
On commercial sites you can find a wide range of longer arrangements of this and other works by Johann Strauss, including the following sheet music for piano: