There are two kinds of present value calculations -- one where you figure the present value of money held in the past and one where you project what the present value of money held now will be in the future.
Money held in the past: To figure the present value of money held in the past, you need a measure of inflation, usually the Consumer Price Index (CPI). To get the U.S. CPI for past years, see "Consumer Price Index." To do the calculations, use the What's a Dollar Worth? calculator posted by the home page of the Federal Reserve Bank of Minneapolis or the BLS's CPI Inflation Calculator. For directions on how to do the calculations by hand, see Understanding What A Dollar Is Worth and/or the What is a Dollar Worth page by the Minneapolis Fed.
See also "Cost of Living."
Money held in the future: To figure the present value of money in the future, you have to posit an interest rate. If you can do that, use the Present Value calculator posted by the University of Illinois at Chicago to find out how much money in the future is worth now. (This is useful if, for example, you want to know how much you have to invest in 8% bonds if you want to get $1 million in 2025).
For more calculators and calculations (e.g., loans), try the MegaConverter, How Much is That? and/or the Inflation/Cost-of-Living section of Mardindale's Calculators Online Center.