About the origin of the silver dollar

The Maria Theresa Thaler is probably one of the most famous and well known coins of the world. See also following links:
http://www.theresia.name/en/
http://en.wikipedia.org/wiki/Maria_Theresa_thaler

Maria Theresia thaler, 1780 Restrike

The Thaler, showing the Habsburg coat of arms on the reverse and the portrait of the Austrian emperor on the obverse, was the currency of the Austrian Empire. It has been minted until 1857, when it was replaced by a decimal guilder. "Thaler" is an abbreviation of "Joachimsthaler", a coin type from the city of Joachimsthal (Jáchymov) in Bohemia.

The Thaler, showing the portrait of the Empress Maria Theresia, has originally been minted from 1740 till 1780. It was very important for trade with the Levant (parts of Turkey, Lebanon, Syria). Over time, the Maria Theresa Thaler became the best known and most popular silver coin in the Arab world. After the death of Empress Maria Theresa in 1780, Joseph II permitted the Austrian mint to continue striking the coin with the 1780 dies in order to meet demand from the Middle East. The 1780 thaler was the only silver coin that the Arabs trusted and would accept. Since then, the Maria Theresa Thaler has been restruck for trade purposes in Vienna, Austria, with the 1780 date frozen in time. The thaler became the unofficial currency in some areas of Africa and Asia, and may still be in use today as a "trade silver dollar" in some Arabian bazaars.

The thaler is 39.5 mm in diameter and 2.5mm thick, weighs 28.0668 grams and contains 23.3890 grams (0.752 troy ounces) of fine silver. After 1858, when the thaler no longer was the Austrian currency, about 300 million restrikes have been minted until well in the 20th century by various mints across the world.

It was one of the first coins used in the United States and probably contributed to the name choice of a dollar as the main unit of currency for the United States.

... and even now you find this 'classic 1780 restrike', being sold at hardly any premium over silver bullion all across the old continent.

A really good thread on the different restrike types can be found at the Coin Community Forum: Modern Maria Theresia Thaler. The Austrian mint has withdrawn minting rights and it currently continues producing and selling the coin only for silver bullion investors.