CThruView Transparent Image Viewer - CThruView is a free transparent image viewer that allows mouse clicks to go through the image.
PiP for Mac - Always on top window preview similar to the popular windows only OnTopReplica
LayerX - An intuitive app to display transparent images on screen.
Pennywise - Cross-platform application to open any website or media in a floating window
Osiva - Osiva" is an acronym that stands for "Overly Simple Image Viewing
Afloat - Create beautiful invoices in 3 clicks, without mistakes