Bedřich Smetana's symphonic poem Vltava (Die Moldau) from his suite Má Vlast. My favourite parts are the two flutes at the opening describing the hot and cold springs which are the source of the Vltava, the main theme, and the section representing the mermaids dancing in the moonlight. The last leaves you feeling that you're watching something on which mere mortals should not set eyes.
Many musical themes resemble the main theme of Vltava including:
La Mantovana (Giuseppe Cenci?), copied here by Smetana
Hatikvah, The Israeli National Anthem
The Czech folk song, Kočka leze dírou ("The Cat Crawls Through the Hole")
I personally think that the Jewish hymn Etz Chayim Hi (It is a Tree of Life) resembles the main theme too.