Previewed - Beautiful mockups & graphics for your next app
CThruView Transparent Image Viewer - CThruView is a free transparent image viewer that allows mouse clicks to go through the image.
Craftwork - A collection of User Interface resources made by Craftwork
LayerX - An intuitive app to display transparent images on screen.
Design Camera - Drag and drop 3D mockups
Osiva - Osiva" is an acronym that stands for "Overly Simple Image Viewing