We all know at this point that Snow Leopard is coming in September. Now you can pre-order it on Amazon.com. To upgrade from Leopard one must only grab briskly at your wallet for a mere $29 for single user and $49 for a 5-license family pack.
Snow Leopard is lacking in the visual advances part, but Apple has greatly refined the very core of the OS. Any Mac user should definitely consider the update especially at the given price.