There are cheap usb controlled power strips, but I have seen several that do not offer any isolation, and if you are creating a possible path for line voltage (120v here in the USA) to your mother board, and its many hundred dollars worth of over-clocked goodness. I would like some isolation.
There are current sensing power strips, One of the outlets is set up to sense current flow. When this happens the electronics in the power strip power on the other outlets. Its a good idea, but sometimes they do not sense correctly, and will not turn on the accesories. Also the electronics require yet another little power supply to be on 24/7, this we are trying to avoid.
There are well designed enterprise grade solutions with isolation, that work very well and have a very hefty price tag as well.
This circuit uses no extra power when it is not in use, and offers some hefty isolation from power surges, and does not cost a fortune to build.
Step 1: What you will need
The heart of this system is two things really, the actual switching is done by a DC controlled solid state relay, All the isolation is provided by a pair of fuses and some transient voltage surge supressing diodes (TVSS)
All of the other parts are really up to you, I used what I had kicking around. Which was mostly standard electrical fittings, and an old plug strip, and a heatsink from a junk processor, and a USB cable that was miss ordered with usb "A" connectors on both ends. Feel free to use whatever works for you.
All told the parts that I had to order (fuses and holders, TVSS, and Solid State Relay) were less then $30.00 USD from an online supplier.