Kuadro was released in 2015 and I discovered it only a few months ago. This proves that I've been living under a rock, or that this handy piece of software is probably not getting the love and attention it deserves.
Kuadro is an isolated executable that can be launched from anywhere on your disk or a remote drive. Download the latest version from the Kruel Games website.
If you don't have a couple of minutes to watch the walkthrough video on the product website, here's the low-down on the features I find most useful.
PureRef is another noteworthy mention, though I find myself using Kuadro most of the time
PureRef is a similar tool, but functions more as a canvas(think Adobe Illustrator's workspace) where you can arrange multiple images, than each image having it's own freely floating window. I found this to be a bit cumbersome on a single monitor setup.
PureRef's biggest plus though, is that it can load all images from a specified URL; an example would be when you want to use multiple reference images from a single Artstation or Behance project.