Back when I started at Microsoft, the big product was MS-DOS 5. Of all the features in MS-DOS 5, my hands down favorite (and one I still use today) was DosKey. You remember DosKey... It was the utility that made F7 one of the most useful keys on your keyboard.
Prior to starting here, I was a MS-DOS 3.x and 4.01 user (depending on whether I was at home or in the lab at school) and became pretty comfortable with the command line interface. To this day, one of the first things I do after booting my PC is to launch a command prompt.
Every so often, someone will stop by my office while I'm working and ask me "What did you just do? That was cool." The "what" was pressing F7 from within a Command Prompt window.
Re-typing short commands is easy and quick, once the commands get lengthy (to me, that means 30+ characters), using the F7 key is a real time saver. Just scroll through the list, when you find your desired command, press the Enter key and you're finished.
This even works inside of many command line utilities (ex: cordbg)!
Happily, DosKey lives on. My F7 key still gets it's daily workout and I can still create and use custom command line macros. For more information on DosKey in Windows XP, check out the docs here. Scroll down for the list of shortcut keys. Scroll a little further for macro syntax.
[Edit: Fix sentence structure]
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