CThruView Transparent Image Viewer - CThruView is a free transparent image viewer that allows mouse clicks to go through the image.
digiKam - Professional Photo Management with the Power of Open Source
LayerX - An intuitive app to display transparent images on screen.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Osiva - Osiva" is an acronym that stands for "Overly Simple Image Viewing
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.