Original links are included though some may have expired.

The Numbers:

Currency trade volume, 1996: ~$1.5 trillion/day
Currency trade volume, 2007: ~$3.0 trillion/day

What They Mean:

A thousand years ago, just as today, a few major currencies served as the world's reserves and mediums of financial exchange. Last autumn, two brothers digging holes on a farm in Gotland -- an island off the Swedish coast, sometimes thought the homeland of legendary warrior Beowulf -- found a pile of 1,100 tenth-century silver coins, weighing about seven pounds and minted in Abbasid-era Baghdad, which evidently served as the source of liquidity for faraway Odin-worshipping Vikings. Another example, buried in Asian languages rather than in peat-bogs: Korea's won, Japan's yen and China's yuan are originally all the same word, based on the Chinese word for a metal disc or coin, and recall the Tang dynasty's role as Asia's financial center.

Financial flows are evidently not new -- it is just that money moves principally by computerized trade rather than by medieval hand or 19^th-century letter of credit. Nor is the large role of a few big currencies much different: 66 percent of the world's identified foreign reserves are in dollars, 25 percent in euros, 7 percent in yen and sterling; and only 2 percent in the world's other 150 national currencies. What has changed are the totals and speeds of currency exchange; and these appear to be growing rapidly. As of 2006, the world's exchange markets were trading $2.7 trillion worth of currency every day; in 2007 the figure should hit $3 trillion a day.

The figure is now roughly double the $1.5 trillion a day traded in 1996, the year before the Asian financial crisis. Some perspective: the $3 trillion figure is ten times the value of daily stock and bond turnover, and a hundred times the value of daily goods and services trade. The United Kingdom is the world's largest currency-trading center, with London accounting for a third of all currency trading, or $1 trillion per day. New York ranks second at about $600 billion a day; Tokyo and Singapore are third and fourth.

Further Reading:

In reserve -- Governments now hold about $4.8 trillion in reserve, three times the $1.5 trillion figure of a decade ago. Some observations:

• The dollar is involved in over 80 percent of currency transactions and remains the world's major reserve currency. The IMF identifies allocations for about $3.15 trillion of this figure, with dollars accounting for 66 percent of the total, euros, 25 percent, yen 3 percent, and sterling 4 percent. China and Japan are the largest reserve holders, together accounting for about $2 trillion of the $4 trillion. Fun fact -- the dollar is named for a small town now in the Czech Republic. It's known as Jakimov today, but until Czech independence the town was called St. Joachimsthale and was the Hapsburg family's big silver mine.
• The big accumulation of reserves is in developing countries, which held $0.9 trillion in 1996 and now hold $3.4 trillion. This reflects cash piled up in Russia and the Persian Gulf through oil sales, along with the determination of Asian countries to build up cash reserves either against the risk of a second financial crisis, to promote exports, or both at the same time.
• The euro's reserve role is growing. European Currency Units, deutschmarks, French francs, and Netherlands guilders combined to make up 17 percent of the world's financial reserves in 1996; the euro hit 19 percent in 2000, and now is at 25 percent of reserves. Yen reserves have shrunk from 7 percent to 3 percent of the world total, while sterling and Swiss francs remain about the same.

A bit surprisingly, no Asian currency has yet begun to take on a role as a reserve comparable to that of the dollar and euro.

Financial reserve data from the IMF, 1995-2006:
http://www.imf.org/external/np/sta/cofer/eng/index.htm

And analysis of the growing role of the euro:
http://www.imf.org/external/pubs/ft/fandd/2007/03/bertuch.htm

Yahoo! tracks the dollar against151 other currencies, from the euro, pound, yen, and yuan to the Samoan pa'anga, the Belarusan ruble, the Comoros franc, the Bhutanese ngultrum and even the North Korean won. The best foreign-exchange bet in the world, for those with good attorneys and/or homes in countries without extradition treaties, is the North Korean won. According to Yahoo! the value of the North Korean won in U.S. dollars is zero. Theoretically a clever person could buy an infinite number of them with a few pennies, then melt them down into metal scrap and waste-paper at a huge profit. The currency is illegal for Americans to own, though:
http://finance.yahoo.com/

International Financial Services, a UK industry association, reports on growth in the for-ex trading markets as of October 2006:
http://www.ifsl.org.uk/pdf_handler.cfm?
file=CBS_Foreign_Exchange_2006
&CFID=481126&CFToken=41537870

Vikings -- Of 150,000 lost and buried Viking-era coins so far dug up in Gotland, 65,000 come from Middle East mints. How did the Vikings get all the silver coins, anyway? A Wikipedia entry, evidently written by a patriotic Scandinavian, insists that they were mainly the peaceful product of economic development, financial integration, and commercial ventures. "The doings of the Vikings put into context are not as savage as they seem," he says, and anyway "others of the time period were much more savage. The image of wild-haired, dirty savages is a distorted picture of reality." Also, the "use of human skulls as drinking vessels is ahistorical," and even "in the 300-year period when Vikings were most active, there were only approximately 347 attacks spread from the British Isles to Morocco, Portugal, and Turkey, and only 430 in Ireland." So put it in perspective. An accompanying photograph shows history-minded Danes re-enacting an early for-ex venture:
http://en.wikipedia.org/wiki/Image:Vikings_fight.JPG