The description of the original helm of teleportation from OD&D recently struck me as particularly interesting:
Helm of Teleportation: The Magic-User employing this helm must have a Teleportation spell in order to take advantage of the device. Having but one such spell the Magic-User can Teleport himself endlessly about the universe, but if he teleports some other person or object the helm does not function and the spell proper is used. Thus the helm is good only to transport the Magic-User himself. Treat as a non-protective helm if worn into combat.
(A passage which also indicates that “protective helms” should have some beneficial effect in combat, but if there’s any explanation for what the benefit would be the rules are rather silent on the matter. I’ve been thinking about applying a -1 AC penalty for missing helmets. But I digress.)
What I was particularly struck by in this passage was the similarity between its mechanical construction and the construction of reserve feats from Complete Mage for 3rd Edition. Conceptually I always liked the idea of reserve feats (allowing spellcasters to make minor magic-based contributions on a regular basis), but found the actual execution to be rather broken. (Allowing wizards to do 6d6 points of area effect damage per round with no saving throw, for example, no longer qualifies as a minor contribution.)
But it might be interesting to take properly balanced reserve-type abilities and have them accessible via magical equipment (like the original helm of teleportation). I’m particularly drawn to the image of magic wands that don’t have charges, but instead allow you to use specific spells you currently have memorized in a powered-down form.
On the other hand, maybe chewing up an equipment slot would be necessary to keep this sort of thing balanced. Or what if there was a percentage chance that you’d lose your reserve spell whenever you triggered the reserve item? In a semi-similar fashion, AD&D’s helm of teleportation limited the number of uses per day based on the number of teleport spells you had prepared. (So that the item extends your magical endurance, but not necessarily limitlessly so.)