Users of the 2017 iMac and iMac Pro get a new Satechi accessory-- the USB-C Clamp Hub Pro, an add-on plugging into one of the rear Thunderbolt 3 slots to add more ports.
Made out of aluminium, the Clamp Hub Pro offers three USB 3.0 Type-A ports with 5Gbps data transfer, as well as an SD card reader. Setup promises to take just a few seconds thanks to an adjustable knob, and the slim design should add little visual bulk to the iMac.