Primarily you can do this just in Bios, as told above.

As a second layer you could just get some smart plugs for your rigs, you can then power on/off remote. We use this solution for remote rigs, it's working nicely if you just have a couple rigs. Our own software - MOAMiner - has online monitoring and offline notification included so we can always check that things run smooth on remote rigs

MOAMiner: https://bitcointalk.org/index.php?topic=5053443