ICTMagic’s Math Links: Loads of links to awesome math tools and sites!
Economic History Net
http://eh.net/hmit/ Calculators for converting a certain amount of money over time. Includes values of old currency and other ways to compare money. Emphasis on England and the U.S. You can look at costs in terms of GDP, unskilled labor, compared to other countries, etc over a wide range of years.
http://www.asknumbers.com/ An online conversion site. Convert weights, distance, currency, computer storage,temperature, cooking, clothing and much more.