NewerTech now has a new adapter that makes it much easier — nay, possible — to mount any 2012-or-newer iMac, even the new Retina 5K iMac with the universal VESA system. The NuMount VESA is a machined aluminum bracket that clamps through the power-cable passthrough in the iMac's stand, adding both a quartet of screw holes for a 100mm x 100mm VESA mount and a shelf above that for storing items you want nearby but neither want to see or need to reach (say, an external hard drive for backups).
The NuMount VESA exists because iMacs purchased since 2012 had to have VESA mounting specified when ordering — it's either an attached stand or four screw holes drilled into the back. So if you purchased your iMac at a retail store, or you ordered it with a stand and now you find you want to mount it up on the wall or an arm, you have an option.
Apple offers a VESA adapter for older iMacs and the aging Thunderbolt Display, but there's no such option to pop off the integrated stand on iMacs made after 2012. So now there's the NewerTech NuMount, although you'll be paying $99 for the privilege.
NewerTech NuMount VESA adapter for iMac - $99.00 - Buy Now