The shortest useful program I have written: %1

This is the shortest program I have written, it is two bytes:

<ExecuteCommand.cmd>%1</ExecuteCommand.cmd>

The task of this program is to execute the parameter as a program, I use this inside Visual Studio on the "Open With"-menu to call cmd-scripts inside the GUI.

The only program that comes close to this is a 5-byte boot program I wrote for 8086 assembly about twenty years ago.