Flashlight - Control your Mac with a keystroke.
Zenity - Zenity is a tool that allows you to display GTK dialog boxes in commandline and shell scripts.
Tiny Flashlight + LED - Tiny Flashlight + LED is free to use application that can be used anywhere else, having nicely built-in functionality.
AutoHotkey - The ultimate automation scripting language for Windows.
Screen Torch Advanced 2 - "Screen Torch Advanced 2", is an handy torch program. It only uses devices' backlight.
Batch MessageBox - Display message boxes from Windows batch files and command line programs.