Energy efficiency is a common way of measuring environmental impact. There are a number of organisations who are involved in measuring and reducing the energy efficiency of computers:
Climate Savers Computing (CSCI) is "a nonprofit group of eco-conscious consumers, businesses and conservation organizations... [who] promote development, deployment and adoption of smart technologies that can both improve the efficiency of a computer’s power delivery and reduce the energy consumed when the computer is in an inactive state." They provide a product catalog of energy-efficient appliances.
Their measures include reference to Energy Star, "an international standard for energy efficient consumer products... Devices carrying the Energy Star logo... generally use 20%–30% less energy than required by [US] federal standards." (Wikipedia)
TCO Certification is a European standard for energy, as well as emissions (including noise), ergonomics and ecology.
(Both Energy Star and TCO Certification provide ecolabels for consumer awareness and certification.)
CSCI also reference 80 PLUS in their impact measurements. 80 PLUS is "an initiative to promote energy efficiency in computer power supply units" specifically.