Apps similar to Skippy-XD include KWin, which is free and open source. Other options are Mission Control, x:choyce, SuperSwitcher and Robata. In total there are five Window Managers like Skippy-XD for Linux and Mac.
KWin is a window manager for the X Window System, and is in the process of becoming a Wayland compositor. It is an integral part, and the default window manager of the Plasma Workspaces, but it can also be used on its own or with other desktop environments.
Mission Control, formerly Exposé, is a feature of the macOS operating system.
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SuperSwitcher is a (more feature-ful) replacement for the Alt-Tab window switching behavior and Ctrl-Alt-Left/Right/Up/Down workspace switching behavior that is currently provided by Metacity.
Robata is a macOS app to quickly see all of your desktop windows, find the one you want, and switch to it.
