I personally use the free Honeyview image viewer.
It has in its General Options the 'Always on top' setting.
I like Honeyview because you can set it up so it has no window border/menu/buttons/anything... then you can just put it right beside whatever it is you're working on:
As you can see it's very unobtrusive... The B&W graphic on the left is open in Honeyview where it stays, always on top of Photoshop.
Another similar option (which requires no other 3rd-party programs) is just to open your reference image in another tab within Photoshop, then drag the tab off as a floating window... Only problem versus Honeyview is the window frame itself taking up real estate:
Alternatively (if you just use Windows Photo Viewer or whatever) you can install Dexpot (virtual desktop manager)... When Dexpot is installed every single window you can open gains the 'Always on Top' functionality.
Here I am setting Photoshop (for example) to be always on top: