Source: Board of Governors of the Federal Reserve System (US)  

Release: H.10 Foreign Exchange Rates  

Units:  Index Jan 1997=100, Not Seasonally Adjusted

Frequency:  Weekly, Ending Wednesday

Averages of daily figures. A weighted average of the foreign exchange value of the U.S. dollar against a subset of the broad index currencies that do not circulate widely outside the country of issue.
Countries whose currencies are included in the other important trading partners index are Mexico, China, Taiwan, Korea, Singapore, Hong Kong, Malaysia, Brazil, Thailand, Philippines, Indonesia, India, Israel, Saudi Arabia, Russia, Argentina, Venezuela, Chile and Colombia.
For more information about trade-weighted indexes see http://www.federalreserve.gov/pubs/bulletin/2005/winter05_index.pdf.