PiP for Mac - Always on top window preview similar to the popular windows only OnTopReplica
Epic Pen - A windows tool for drawing over your desktop and applications
PiPer - A DIY Minecraft computer to build gadgets
gInk - An on-screen on-screen annotation software for Windows.
CThruView Transparent Image Viewer - CThruView is a free transparent image viewer that allows mouse clicks to go through the image.
Gromit-MPX - Create annotations by directly drawing on your screen.