Remote power management of computers is becoming an increasingly powerful tool to control energy consumption and carbon emissions. Behavioral techniques, like encouraging users to turn off their machines at the end of the day, are not always effective and may interfere with off-hours IT activities like upgrades. The Energy Star program now publishes a list of commercial and open-source power management products, along with success stories. Visit Energy Star's Power Management home page for an overview of resources for both organizations and individuals.

OS-specific resources include:

Open4eneregy offers an open source application called energyManager for building a baseline energy profile.

Update 20090514:
Those interested in the future of power management can access presentations from SciPM 2009: Workshop on the Science of Power Management. (Thanks to Ken Christensen for alerting me to their availability.)

Update 20090922:
Open4energy now offers a comprehensive listing of tools for data center energy management. It's a good place to share your experiences with these products and vendors.