It depends on your hardware. If it is a server, you probably can use some remoting feature for that (like
iLo[
^] on HP). Consult with your server vendor.
On some desktops there is
vPro[
^] that can be also used to control power state.
But these are not available for all hardware platforms. So if you are at the start of a project, try to purchase hardware with such remote control feature. If you want to use exisiting hardware without such feature, than you will need to purchase network controllable switch like OriginalGriff suggested.
Submit an article or tip