The "forever stamp" doesn't currently exist, but it's a new idea by the US Postal Service made to the independent Postal Rate Commission in early 2006 as part of their proposal to increase the price of sending first class letters. The forever stamp was proposed as a way to help individuals and business hedge against postal rate increases in future years.

Basically, the idea of a "forever stamp" is that once the stamp is purchased, it can be used to send a first class letter at any time in the future. Even if stamp prices increase in the future, the forever stamp would still be valid for sending a first class letter without paying any additional money to cover the increases.