I just found another one, and one last year. If you're listening Susan (since over 4 years ago) give us an update...

Polish Lullaby (Lulajze Jezuniu), a Christmas Carol:
This melody was a Polish folk tune and adapted by Chopin in his Scherzo in B minor, opus 20.

Also, Schubert's Adagio in E flat, D. 897, Notturno, (a piano trio) is from an Itallian folksong - wish I knew the name - where the melody phrase starts on the 2nd beat! (according to Karl Haas)