According to Sust-it.net, an energy website that ranks energy usage and cost for thousands of home appliances, entertainment devices, and computers, Apple's Mac mini is the most energy efficient of all the computers on the market.
With its 2.0 GHz Intel Core 2 Duo, the Mac mini uses .94W when powered off, just 1.71W when it's in sleep mode, and 12.6W when turned on but idle. According to the website, the energy cost per year (sans monitor) is $5.51 when calculated in accordance with the average United States electricity rates. While this particular model is no longer made by Apple, the new 2.26 GHz is and it's even more efficient. Apple's environmental report for the mini shows the revised model uses only .80W when powered off, 1.39W in sleep mode, and 13.4W when idle.
As far as other "green" computers on the list, Apple computers (both laptops and all-in-ones) were all ranked pretty high. The 1.86 GHz MacBook Air comes in at second place in the laptop category behind the HP Pavilion DM3. Both the laptops cost around $2.39 per year, but the HP had lower power consumption when powered off and in sleep mode.
As for the all-in-ones, eMachines, Acer, Asus, and MSI all came in the top five spots, followed by the iMac at number six.
If you're looking to make a smaller carbon footprint and buy a greener computer, the list at the sust-it website is a good place to do some comparison shopping. Though it's not apparent how often the list is updated, just make sure you know that the computer world is ever-changing and tomorrow, Apple or any other company could come out with an even greener PC.