Original links are included though some may have expired.

The Numbers:

World stock market capitalization, December 2007: ~$63 trillion
World stock market capitalization, January 22, 2008: ~$50 trillion?

What They Mean:

Looking back upon the 1920s in his book The Great Crash, John Kenneth Galbraith wrote:

"Never before or since have so many become so wondrously, so effortlessly and so quickly rich... Perhaps it was worth being poor for a long time, to be so rich for just a little while."

In the last decade, fewer have briefly gotten rich. In the next, it is to be hoped, fewer will be poor. That said: At the end of 1997, in the midst of the Asian financial crisis, world stock market domestic capitalization (i.e. of local firms only, to avoid duplication for firms listed on several exchanges) totaled $21.6 trillion. This was about 70 percent of the year's $30.1 trillion world GDP. The International Federation of Stock Exchanges' report for December 2007 placed the domestic market capitalization of its 51 stock exchange members -- from the little exchanges in Bermuda, Colombia, Malta, and Mauritius at $3 billion to $7 billion apiece; to the gigantic New York Stock Exchange at nearly $16 trillion -- at $63 trillion, or 110 percent of the $57 trillion global GDP. Two observations:

• In 1997, NYSE's $10.3 trillion in domestic market capitalization was almost half of total world domestic market capitalization. The $16.3 trillion figure for 2007 is much bigger, but makes up only a quarter of the world total, as European market capitalization has tripled and there has been six-fold growth in Asia. For the record: After NYSE come the London, NASDAQ, "Euronext" (combining the Amsterdam, Paris, and Brussels exchanges, and owned by NYSE since 2006) and Tokyo exchanges, each above $4 trillion. Then Hong Kong and Shanghai at $3 trillion each, and Germany and Toronto at $2 trillion. Switzerland, Stockholm, Milan, Madrid, India National, Bombay, Australia, and Sao Paulo all came in between $1 trillion and $2 trillion.
• Between late December and yesterday's Federal Reserve interest rate cuts, most major stock indexes had dropped by more than 10 percent, and several by 20 percent. NYSE's S&P 500 was down from 1500 to 1310, or by 13 percent, and London's FTSE was also down by 13 percent. Germany's DAX is down 15 percent, Tokyo's Nikkei is down 20 percent, and Hong Kong's Hang Seng 23 percent. Using these indexes as a rough approximation of total value, a 15 percent drop in market capitalization worldwide would be a loss of about $9 trillion. This is about equal to the combined annual GDP's of China, India, and Southeast Asia combined; or the United States minus the southern states; or the combined GDP's of Germany, Britain, France, Italy, and the Netherlands.

Further Reading:

What to do? Interest rate cuts and stimulus packages for growth; PPI's Paul Weinstein has ideas for emergency support for Americans with sub-prime mortgages:
http://www.ppionline.org/ppi_ci.cfm?kaid=125
&subid=162&contentid=254479

Count how much money you had, last month -- the World Federation of Exchanges has market capitalization, equity, etc. statistics up to December 2007:
http://www.world-exchanges.org/WFE/
home.asp?action=document&menu=10

Crisis literature -- The worst financial crisis of recent memory, Asia's 1997-1999 siege, has left little memorable English-language literature. Nor have the dot.com collapses a few years later produced much beyond journalistic accounts and rueful memoirs. The classic financial-crisis reading list:

• For analysis -- Charles Kindleberger, a designer of the Marshall Plan in the 1940s and the premier explainer of economies going really badly wrong. His Manias, Panics and Crashes analyses and compares nearly 20 international financial crises, from the Tulipomania in 17th-century Holland to the dot-coms:
  http://www.amazon.com/Manias-Panics-Crashes-
  Financial-Investment/dp/0471467146
• For anecdote -- Galbraith's The Great Crash 1929 informs us, among much else, that very few stockbrokers (perhaps only two) actually jumped off of window ledges in 1929 and 1930:
  http://www.amazon.com/Great-Crash-1929-
  Kenneth-Galbraith/dp/0395859999
• For story -- John Carswell on the South Sea Bubble recounts the largest crisis of them all -- the 1720 South Sea Company stock fraud which created the term "bubble;" it meant $107 million pounds in losses, at a time when analysis of British GDP yields figures in the 50-75 million range -- and for good measure the nearly simultaneous crisis in Paris, when fugitive murderer and gambling genius John Law wrecked the French economy by introducing paper money:
  http://www.amazon.com/exec/obidos/ASIN/0862999189/
  ref=nosim/kipnotescom-20
• For status -- Charles Mackay's 1842 Extraordinary Popular Delusions and the Madness of Crowds is the classic of the genre:
  http://www.econlib.org/library/mackay/
  macExContents.html