Well, as Harry states depends on your intentions and whether you have USB 2 or 3 ports on your Mac - so give us some more details.
USB buses can be 'stand alones' or be 'powered', the latter providing more amperage per port, depending on the USB version (typically USB 2.0, 0.5 Amp vs. USB 3.0, 0.9 Amp); also, the speed varies w/ the type of USB standard chosen, so a number of decisions to make.
I've used non-powered hubs which are fine for low amperage purposes, such as flash memory sticks and printers; external HDs are more difficult - I was doing my Carbon Copy Cloner backups for my iMac on a powered hub w/o a problem - then bought my wife an iMac stand w/ built-in non-powered hub, the external HDs would not consistently work; also, my Apple SuperDrive (optical) seems only to work when plugged directly into the computer's USB port (whether on my iMac or MBPro).
Thus, provide us some information regarding why you want a USB hub, what USB standard(s) you are using, and what devices you need to attach. Dave
Thanks for your help. Don't really think I need one now,only have external hard drive and apple SuperDrive plugged in as printer is wifi. Just like buying kit. Have to think of something else now.
Best wishes to everyone for 2015
OK - so you do not want a hub? Another option if you occasionally plug in several different kinds of devices to a USB port on the back of the iMac (which can be a pain) is to obtain an inexpensive Male-Female USB extension cable - I bought the one shown below - USB 3.0 and just $5 from Amazon - used for a variety of plug-in purposes, and easy when I do my CCC (Carbon Copy Cloner) backups on two different external HDs (like redundancy - ) Dave
Those short USB extension cables I bought for our iMacs are one of the handiest and best convenience things I've ever bought — and a whole $3.00cdn each for good quality ones from our Dollar Den store just a very short drive away!!