I wrote about this a while back, and it seems someone had the exact same idea. Brando just released a USB hub with switches that let you turn the connected devices on or off as you please.

http://usb.brando.com.hk/prod_detail.php?prod_id=00394

I’m not too fond of the design (which does little to hide the wiring and uses slide switches instead of more convenient push buttons), and a couple of more ports would have been nice, but this is a great device nontheless. Every time someone does not power up that little-used scanner and saves rain forests as a result is important.