Joulemeter from Microsoft Research is a software based mechanism to measure the energy usage of virtual machines (VM), servers, desktops, laptops, and even individual software running on a Windows computer.
Joulemeter estimates the energy usage of a VM, computer, or software by measuring the hardware resources (CPU, disk, memory, screen etc) being used and converting the resource usage to actual power usage based on automatically learned realistic power models.
Joulemeter can be used for gaining visibility into energy use and for making several power management and provisioning decisions in data centers, client computing, and software design.
It tracks computer resources, such as CPU utilization and screen brightness, and estimates power usage.
Go here for more: Product Information.
UPDATE: The public release of Joulemeter is deprecated and it is no longer available for public download. Similar energy estimation capability is now available through Visual Studio.