I recently installed the Granola energy management software to my netbook just to see if it would eek out some extended battery life time. Initial results look good and battery time has indeed been increased with no apparent effect on perceived speed or response.
Granola is available for linux and Windows desktops. It can also be deployed on VMware ESXi. It is free for personal use but has a very small cost licence which makes available much more detailed energy controls and monitoring options when deployed across many computers at the same address. Power saving results can be tracked over time via an online account which records your power usage data.
Available desktop stats are displayed in a slightly generalised way with preferences available to show annual or cumulative energy savings.