Genealogists often find references to money in old deeds and other documents. Even U.S. census records frequently recorded estimates of a person's real estate. The naturally question is, "I wonder what that would equal in today's dollars?" There is a web site that can answer this question.
S. Morgan Friedman' Inflation Calculator can convert a U.S. dollar amount for any year from 1800 through 2008 into the equivalent amount, adjusted for inflation, in any other year of that range. In other words, if you find that your ancestor purchased land for $400 in 1805, the Inflation Calculator will tell you that the money he spent is equivalent to a purchase of $5667.99 in 2008.
This should be sufficient for genealogy purposes. The pre-1975 data comes from the Consumer Price Index statistics published in the Historical Statistics of the United States (USGPO, 1975). All data since then is from the annual Statistical Abstracts of the United States. You can access the Inflation Calculator at: http://www.westegg.com/inflation.
Canadians will find a similar Inflation Calculator for the years 1914 through 2009 at the Bank of Canada's Web pages at: http://www.bankofcanada.ca/en/inflation_calc.htm.