KeyCastr, an open-source keystroke visualizer.
brew cask install keycastr
In order to work, KeyCastr must be included in the list of applications with Accessibility API access, under the Security & Privacy system preferences.
To add KeyCastr to the list click the + button and select KeyCastr from the file system.
The default position is on the bottom left of your display. To modify the position of displayed keystrokes, click and drag the text like so: