SmartPower is particularly suited to Windows Home Servers, torrent/download boxes and HTPCs.

One or more schedules are active - A schedule is a period of time during which the PC must stay on. By default SmartPower will wake-up your PC in order to honour a schedule.
One or more devices are online - A device is anything on your network that will respond to a ping.
Network throughput (the bytes flowing through a particular interface) is above a configurable threshold.
CPU usage is above a configurable threshold.
One or more processes are running - A process is a Windows process.
Download the installer using the link at the top of this page.
Run the installer and follow the instructions.
When the installer has finished, click Start, click Programs, click SmartPower, click SmartPower Configuration.
The SmartPower configuration utility will open and will ask you if you want to start the SmartPower service. Click Yes. (You will only need to do this once; when your PC next boots the service will start automatically).
Configure SmartPower as required and click Save. You changes will take effect immediately.
Disable hibernate/suspend on system idle in your power saving settings. SmartPower cannot (currently) wake-up your PC if Windows has hibernated/suspended it.