Trigraphy - Photo editing app that allows you to make mosaic/abstract looking art.
Wallpaper Engine - Wallpaper Engine enables you to use live wallpapers on your Windows desktop.
Dreamscope - Make and share 'deep dream' images using dozens of supplied styles or your own.
Lively Wallpaper - What is it? Lively is a Free and Open-Source Software (FOSS) for animated desktop wallpapers.
Photomosh - Turn anything into a vaporwave
ScreenPlay - Open-source and cross-platform Wallpaper, Widgets and AppDrawer app.