If you're ever a contestant on Jeopardy or find yourself in the "cash cab" in New York, you'll be relieved to know that you can answer any stamp-related questions ...
Non-denominated stamps do not show a monetary value, or denomination, on the face.
Typically, the Postal Service issues non-denominated stamps to cover the period immediately following a First-Class Mail price increase. Since USPS needs to maintain a sufficient supply of stamps to meet customer demand, non-denominated stamps allow USPS to print hundreds of millions of stamps even when new prices are uncertain.
The first non-denominated stamps were two Christmas stamps issued in October 1975. In 1978, the Postal Service began to put letters of the alphabet on the stamps, beginning with “A,” to help customers keep track of the stamps’ value. After reaching the letter “H” in 1988, this practice was discarded in favor of simply indicating the class of postage (for example, First-Class Mail) for which the stamp was intended.
Non-denominated postage stamps and their values: A = 15 cents, B = 18 cents, C = 20 cents, D = 22 cents, E = 25 cents, F = 29 cents, F rate make-up stamp = 4 cents, G = 32 cents, G rate make-up stamps = 3 cents, H = 33 cents, and H rate make-up = 1 cent.
On April 12, 2007, the Postal Service changed the nature of non-denominated stamps “forever” when it issued its Forever Stamp. Like other non-denominated stamps, the Forever Stamp is always sold at the current First-Class Mail postage price. But unlike traditional denominated stamps — that retain only their original value after a price increase — the Forever Stamp always remains valid for single-piece First-Class Mail postage regardless of any future price increases.
Although most denominated stamps were for First-Class Mail postage, they do come in many values, including many for presorted Standard Mail. Also, USPS semi-postal stamps are non-denominated. The price of a semi-postal stamp pays for the First-Class Mail single-piece postage price in effect at the time of purchase, plus an amount to fund appropriate causes that the Postal Service determines to be in the national public interest. The Breast Cancer Research semi-postal stamp — first semi-postal in U.S. history — was issued July 29, 1998. It still sells today. As of September 2010, it has raised nearly $71 million for breast cancer research.
Source: Liteblue.usps.gov "Beyond the Blue"