How do I enable power management on Ubuntu Server?
There is the manual method (using shell scripts) but I do not know of it. This software seems to be able to do the job (http://www.ubuntugeek.com/micromiser-power-saving-software-for-ubuntu-laptopsdesktopsservers.html), but it has been renamed (it seems) to Granola.
So, use Granola from here: http://grano.la/support/linux_install.php?download&os=linux. Just download the installer.bash file and follow the instructions on the same web page to download and install granola.
NOTE: This URL is no longer valid, however it was left alone for historical reasons by moderators.