I use my Logitech G13 for my game controller, and it has a pretty cool configuration utility that shows you the keys and allows you to map your controls. I was using it for a reference for a while. Then I purchased more Logitech peripherals, and they worked great with the utility, other than the fact that my mouse is wired/wireless, and whenever I plugged/unplugged it, the stupid app would automatically switch to the mouse- very inconvenient if I do this in a game. Also, having the ability to bring it up as an overlay would have been nice.
Enter the Reference Overlay Tool. It does one simple thing- points to a directory of images, and displays them in a transparent window.
I have several other features planned from my own use of the software, namely:
A hotkey to bring the window to the front
An always on top option
Support for other image types other than .PNG
The ability to change the background color
The ability to lock the image on the fly to make sure it is not changed by accident
The ability to link the display of the image in the list to a running program so that when that program is run, that image is automatically displayed.
The ability to hide but not close the application, so it can monitor for applications run, and listen for the hotkey to come to the front.
More information, including more extensive documentation is located at http://www.thinkshui.net/software/reference-overlay-tool/.
Note that if you paid money for this software, ask for it back!