The current price of a US stamp is:  

44 cents

as of
May 11, 2009: 
USPS stamps are 44 cents.
First-Class Mail Letter (1 oz.) is 44 cents. (Each additional ounce costs 17 cents.)

You can buy "forever stamps" at the current 44 cent rate, and you won't have to pay for additional stamps the next time prices go up!

From United States Post Office's stamp prices (deeply buried, which is why we post the price here!  :P)