In Sweden, the United Kingdom, France, Canada, Belgium, etc., stamps are sold with non denominated values printed, but that can be always be used for a certain service : generally the first rate of the prioritary letter.
In the United States, these for-one-service value stamps exist and are useful : you don't have to remember the exact postal rate. However, their monetary value don't evolve : it is the value of their issue. After two or three rate changes, the expeditor can be puzzled when the USPS bring back one of his letter, estimated underfranked with a "first class mail" stamp.
That's why the Forever stamp was in May 2007 a postal revolution in the United States : finally, the federal Congress authorized the existence of a stamp whose monetary value will always suffice for the simple letter postage rate.
How can our US citizen know if he can stick alone his old "first class mail" stamp ? The Virtual Stamp Club found a help page by the USPS (pdf file).