EDIT Currency
Dollar is the name for the currency of various countries, but when used without a specific country name set in front it usually refers to the US Dollar. Other than the USA it is also the official currency of the Federated States of Micronesia, Ecuador, Palau, and the Virgin Islands.
Since 1945 it is also the official international key currency. The US-Dollar also often serves as a kind of sub-currency in many countries where the own currency is weak such as Cuba.
Other national Dollar currencies are the Australian dollar, Barbados dollar, Bahamian dollar, Belize dollar, Bermuda dollar, Brunei dollar, Canadian dollar,Cayman Islands dollar, East Caribbean dollar, Fijian dollar, Guyanese dollar, Hong Kong dollar, Jamaican dollar, Liberian dollar, Namibian dollar , New Zealand dollar, Singapore dollar, Solomon Islands dollar, Suriname dollar, New Taiwan dollar, Trinidad and Tobago dollar and the Zimbabwe dollar.

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EDIT History
There are various explanations for the origin of the dollar sign. The U.S. Bureau of Engraving and Printing states that the dollar sign evolved out of the Spanish "Ps" for pesos (or "P8" piastres, pieces of eight, as one peso was made up of eight reales which is another Spanish currency). This abbreviation occured first in Mexico in 1696 and was only common in the Spanish colonies, never in Spain. The flying s then gradually moved on top of the P which became more and more of a single stroke. This sign was widely used and it is quite likely that the American Congress adopted this currency sign upon the creation of the United States Dollar in 1785. The first time that the Dollar sign appeared printed in a book and as official American Dollar sign was in »American Accomptant« by Chauncey Lee, New York 1797.

Other theories say that the double-stroke dollar sign evolved out of the superimposed U and S stamped on the currency bags by the US mint. The U would have lost its bottom bow at some point.
Another possible origin are the Pillars of Hermes (Hercules). This is an ancient name of the promontories on the entrance of the straits of Gibraltar, and when King Ferdinand was able to make Gibraltar part of the Spanish estates in 1492 he adopted the symbol of the pillars of Hercules. Later, King Charles V used it in his coat of arms and the symbol in combination with two hemispheres was printed on coins made of the silver and gold that was brought from America by the counquistadores. These coins where then spread in America and Europe and the symbol adopted as a currency symbol.

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EDIT Name
The name is related to various old currencies in Europe: Tolar in Bohemia, Thaler /Taler in Germany, Daalder in the Netherlands and Daler in Sweden, Denmark, and Norway. The name thaler originally came from the German Guldengroschen, minted from the silver of a mine at Joachimsthal (St. Joachim's Valley) in Bohemia (then part of the Habsburg Empire) by the men of the von Schlick-family since 1518. The name Spanish dollar was used for the peso, a Spanish silver coin, which was widely circulated during the 18th century in the Spanish colonies in the New World. Dollar as a name for Thaler was common in the English language already since the late 16th century.

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EDIT Number Sign
Around the year 1500, the Portuguese used a symbol that represented the number 1000 and was very similar to the $. It was called cifrado. At that time, both the roman and the arabic ways of writing numbers were common and the cifrado was used with both.

12.410 in the Portuguese/Roman way: XII $ CDX
in the Arabic way: 12 $ 410

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EDIT Informatics
the $ often denotes the beginning of hexadecimal numbers ($A000) or the beginning of a string (the succession of characters) in older versions of the computerlanguage BASIC. It is also used to define variables in Perl and PHP programming languages, and in most Shell scripting languages.
$$$ is also the extension of some temporary files.

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