there are a lot of NAS systems that meet your criteria. F.e. NetGear ReadyNAS, QNAP TSx09, Synology diskstation. And if it has to be really cheap, look at Linksys NSLU2 or Lacie.
Most of these systems power down the harddisk(s) when not in use, so why would you want to switch it off remotely ? )
http://www.komforto.com
enjoy the digital lifestyle