PiP for Mac - Always on top window preview similar to the popular windows only OnTopReplica
Helium - Helium lets you accept payments by credit card for anything you want to sell online.
CThruView Transparent Image Viewer - CThruView is a free transparent image viewer that allows mouse clicks to go through the image.
PiPifier - PiPifier is a macOS 10.
Afloat - Create beautiful invoices in 3 clicks, without mistakes
X-Mouse Button Control - XMouse Button Control (XMBC) allows you to create application and window specific profiles.