Cantrip is an archaic word originally meant to define a trick performed by witchery. Later it was used to mean a malicious trick or a sham. In recent years (at least since the publication of the AD&D version of "Unearthed Arcana") it has come into usage in fantasy role playing to mean a bit of minor magic. This is typically in the form of 0 level spells in D&D and it's simulacra. I like the idea of a magic user having access to spells that are useful little utilitarian spells that do not have major effects. That said, I don't believe that these effects should be individual spells themselves. I like the idea that a Vancian mage could memorize a spell and siphon off some of it's power without releasing the spell. This does not work with divinely granted magic like that of a cleric or druid.
How are we going to make this work? Well, this takes bargaining between the referee and the player. A mage can attempt to cause a minor magical effect based upon the effect of any spell currently memorized (or known spells not expended in the case of my sorcerer class.) For example, a mage with fireball written may want to light a pipe from his finger. A witch being bothered by a would be suitor may want to make him drowsy, heading off to bed, without expending the full effect of the spell and making him not off there and then. Likewise a sorceress with telekinesis may want to cause a bell that she can see ring, or wind chimes sound, without using the full effect of her spell. Minor effects like this might be allowed without question (possibly allowing a saving throw for the chap that is getting drowsy.)
Occasionally a player may want to make a cantrip a little more powerful. If this is allowed it should only be allowed once prior to expending the spell, and be possible to fail at. For an example, let's look at the fellow who lit his pipe with a fireball based cantrip. He may want to use that fireball for other things. He may want it to deal damage with it, to a lesser extent, than normal. Perhaps he is in close quarters and wants to reduce it to a 1d6 "firebolt" cantrip focusing on one enemy? Or maybe he and his companions are in a cold region and cannot light a fire. In this instance, perhaps he wants to use it to generate heat? These things are more game affecting than using the cantrip to light a fire or pipe. The referee can, of course, just rule that these sorts of things are not possible. Alternatively this could be allowed, but making an attribute check based upon the caster's prime requisite. My Power attribute could be substituted, if it were being used. If the check were successful, the effect could go off. Note that as stated before, more powerful cantrips should be of limited use. If the check fails, however, a saving throw should be made at a penalty of the spell's level. If the save passes, the effect does not occur. If the save is failed, the spell is expended for the day as if it were cast normally.
Let me know what you think. I've been playing around with this for a while.