This brushed aluminum USB hub by Satechi clamps onto the bottom of any 2012-or-later iMac and plugs into one of the rear USB ports, granting you convenient access to four front-facing (and super fast) USB 3.0 slots. To attach the hub, simply unscrew the screw on the back, place the clamp's rubber grips on either side of the iMac monitor, and rescrew the clamp.

If your iMac happens to be a pre-2012 model, one Amazon reviewer has a handy tip for you:

[...] you are not completely out of luck. You can completely remove the bracket and place a strong piece of adhesive tape of the top of the hub to hold it in place underneath your iMac. It wont be as secure as with the clamp, but it does allow to have the four ports easily accessible.

Get the clamp + hub for $30 on Amazon.