The USPS First Class postage stamp’s current price is $0.49 as of January 24, 2014.
USPS stamps are for normal-sized letters weighing one ounce or less within the USA.
The old price of stamps before January 24th was $0.43 per stamp. If any previous stamp you purchased was a Forever Stamp at the old price of $0.46, then they are valid and still can be used the same as $0.49 stamps.
While you can place two stamps on a letter weighing less than 2 ounces, stamps are not cost effective beyond 1 oz. letters as First-Class shipping labels become a more cost effective service.