In a recent CNET review, editors found that Apple's Snow Leopard OS is green. Well, sort of--they estimated that a MacBook with Snow Leopard installed would use about a dollar's worth less electricity than with prior Apple OSX operating systems. That may seem like a paltry sum, but when you consider that Apple sells about 10 million computers per year, that dollar adds up quickly.
While Windows 7 is also said to be more energy-efficient than its predecessors, it won't be released to the public until October; Apple's Snow Leopard OS is available now. Looks like Apple has beaten Windows to the green punch--at least in this respect.