Cantrips! Zero-level spells. I remember when they first came out in Dragon magazine and found them fascinating, they were something of a more practical use of magic, with spells to clean and dust and pull pranks. But to actually use them in your AD&D game, your MU would have to give up a 1st level spell, and frankly, there's just too many useful 1st level spells for a MU to do that, even at mid to high levels. So it was basically a neat idea that was quickly forgotten, except in special cases (like 0-Level characters).
Cantrips did make a reappearance in the 3rd edition of the game (and in 5th), where they became spells cast in addition to 1st level spells, and some of the crappier 1st level spells were shifted downward (such as light and mending). It actually went a long way to making 1st level (or even 2nd) MUs be more useful, but they largely escaped a re-emergence in the OSR. At least until now, with Cantrips: Supplemental Magic Rules for OSR Roleplaying Games by Jeff George, which is $1.99 for 32 single column pages and usable for all major flavors of OSR.
