Dreidl is a children's game, played with a spinning top, that is universally associated with Chanucah. A popular legend ascribes its origin to the Jewish children at the time of the Hasmonean revolt. The evil Antiochus forbade the Jews to study Torah.
To evade the decree, Jewish children hid and studied in caves. When discovered, they would conceal their books, whip out their dreidls and pretend to be playing a harmless game.
This is a pretty story, consistent with the timeless meaning of Chanucah, but with no basis in historical fact. It appears that dreidl was a medieval German gambling game. (The word itself comes from the German, trudeln, meaning to spin, or possibly drehen, to rotate.)
In the original version, the four sides of the top were marked with the letters n, g, h and s, standing for nichts (nothing), ganz (take all), halb (take half) and stell ein (put into the kitty).
In Yiddish/Hebrew, these letters became nun, heh, gimmel, shin. This was taken to stand for ness gadol hayah sham, meaning a great miracle happened there, that is, in the land of Israel.
Consequently, dreidls in Israel replace the letter shin with peh, to make the phrase ness gadol hayah poh, or, a great miracle happened here.
Dreidl is thus a beautiful example of how Judaism can absorb an outside practice and imbue it with Jewish significance.