Thanks for all of the replies. One problem I have is updating a Road Angel. My existing USB ports don't supply enough power to get the job done. I need to connect the device to a 12V supply to enable me to synchronise it's on-board database with that of a server. I am hoping that I can obtain a hub which will supply enough power to complete the process without having to 'power up' each time.
Also, over the years, I have acquired and use quite a few USB devices which has meant using up the two USB sockets on the front of my machine, so when I need to use a pen drive, I've got to disconnect another device first.
Perhaps I don't fully understand the situation, but as far as I am aware, each USB port should be capable of supplying 500mA. The mains adapter mentioned in woodchip's otherwise excellent post seems only to supply a total of 1500mA between 7 ports. This will probably be OK if you only pull full power from 1 or 2 ports, but if you need more, then the adapter supplied may start to wilt!
As corroboration, my Belkin adapter supplies 2100mA for 4 ports....