Riu Riu Chiu is a Spanish song from the 1500s. Sung below by The Monkees (via Caffeinated Joe):
It is widely performed by many types of musicians, including
The Waverly Consort, the Boston Camerata, the Kingston Trio, Mirabilis, the Modern Folk Quartet, the Baltimore Consort and Sixpence None the Richer.
You can read the lyrics and an English translation here.