As I recall, the "magic" of the Deryni books was closer in spirit to psychic powers than most other types of magic.
Game-wise, I like Green Ronin's "The Psychic's Handbook", which fed into Blue Rose and expanded into the True20 RPG. "Psychic" abilities were subtler than spells both in casting and effect ... less powerful, but you could keep it up all day if you continued to concentrate and the effect didn't fatigue you too much. Yes, a fireball could take out a huge number of orcs, but a "psychic" can worm his way into the real seat of power ... or mind-boogie the orc leader to sound a retreat.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)