While the Beit HaMikdash (Holy Temple) was standing, Jews donated a half-shekel yearly to the Temple. This money paid for cattle for communal sacrifices, and served as a way to count the nation, since taking a direct census was forbidden. The tax was paid during the month of Adar, before the Fiscal New Year, Rosh Chodesh Nissan. Thus all the supplies could be purchased in time for the New Year. Even after the temple was destroyed, Jews continued send this donation in from the Diaspora to support the Talmudic students in the Land of Israel.

In the Talmud, Resh Lakish said this payment was proscribed since,
"It was known to the Almighty that Haman would someday pay shekels for royal permission to destroy Israel. He therefore anticipated the shekels of Haman with the shekels of Israel. (Megilla 13b)

Tradition tells us that this mitzvah, and the merit of the unity that it implies, was one of the factors (in addition to the Jews' sincere repentence) that helped deflect the designs of Haman, and bring about his downfall.

Today, throughout the world, Jews continue to donate a symbolic amount to charity in the synagogue, usually on the afternoon before Purim. The custom is to donate three coins, in the value of half the denomination in the country. In the U.S. people donate half-dollars. In Israel, half-shekels. In some places, silver coins are used.

But don't forget -- you have to give tzedaka (charity) on Purim itself! Give your Matanot L'Evyonim- Gifts to the Poor!