Ghostie
I've been working on more Mac side projects lately. Here's an app I made to scratch an itch: Ghostie is just a window that shows an image. You can set the window to float on top of other windows and adjust the opacity with a slider. I use it to keep reference images in the corner of my screen so I don't have to switch back and forth between spaces.
Source on Github, do-want-you-want license.