I use wake on lan function. PC goes to sleep except for keeping the network interface powered up. Network looks for specific WOL packet and if it finds it triggers the BIOS out of sleep mode and PC boots. Needs either a special NIC that has connector for lead to motherboard (not in our case) or more easily use an integrated network interface on the motherboard (we use Intel 915 with this).
End of the day it saves power (look at the back of a "switched of PC" with this feature enabled and you will see the network socket LEDs lit up as this part is still powered.