For historical time series:

• Global Financial Data: Search for ‘exchange rate’ and select a currency. 
   
• Bloomberg (MIT Sloan only): Type Exchange Rates and hit the HELP key. Select Currencies. Or, hit the Currency Market Sector key and then hit GO.
   
• Datastream (workstations 5 & 6  in Dewey Library): Search under Exchange Rates
   
• WRDS: Select Federal Reserve Bank
   
• Federal Reserve Statistical Release: Foreign Exchange Rates
• FRED page on exchange rates

For currency calculators:

• OANDA.com
• Bloomberg Currency Converter