Screenkey is an open-source, cross-platform app and utility that allows users to display their keystrokes on the screen in real time. It is designed to provide a visual representation of how users interact with their keyboard and can be used to help teach and demonstrate keyboard shortcuts, typing techniques, and other keystrokes. Screenkey can also be used to debug programs, troubleshoot user issues, and help provide visual feedback during presentations. The app can be used on both Windows and macOS, as well as on Linux using the X Window System.
Carnac
Screenflick
KeyPress OSD
Key-mon provides a small window that also displays the mouse usage status.