You mean like a printer? I think that each usb port needs so much voltage to run them don't they?
I also have a pcmcia card that has only two usb's in it and in the middle of the device is a place to plug in power as well.
A USB port can supply up to a certain amount (I think 500mA) of power. If you connect devices to an unpowered hub which use less than that in total, it's fine -- the hub can just share the power from the PC. If your total requirement is more than that, you'll have issues unless the hub has its own power supply.
Same as any other piece of equipment -- use whatever voltage and current rating the device is expecting. With any luck the label on the hub will tell you what it needs, or else you might find out at the manufacturer's web site.