Faradon
Fantasy Hero / Hero / Champions or whatever you want to call the game system...
I like it mainly for the fact that the entire game, to include the magic system, is based on a levelless point buy system.
I'm with you. HERO, in all of its incarnations, but especially 4th and 5th editions, allows you to model any kind of PC you want.
This means modeling any kind of magic system is a cinch:
You want to have a Vancian style PC? Design and determine the cost of your spell, and limit its casting to X/Day.
You want a D20 Sorcerer? Design a Magic Pool (probably Multipower), with enumerated spells, and he can use any one of them up until he runs out of power...
You want a WOTC Warlock with only a couple of spells that he can use repeatedly? Design the spells, and he can use it as he sees fit- depending on design, an infinite amount of times.
Elementalist? All the spells have fire (or electrical, or cold, etc.) effects in his Elemental control.
You want a spellcaster who makes things up on the fly? "Cosmic" Variable Power Pool is your tool.
Runestone casters? All of his spells have physical foci that he must have to cast spells.
Psionics? No problem- model them any way you want- weak or strong; wild or disciplined.
Ritual Magic? Just lengthen the casting time.
PCs with magic abilities due to curses, prophesies, or otherworldly heritage?
Tattoo Magic? Imbed the spells in the PC either as merely a power, or as an Obvious/Inobvious Inaccessible Focus.
Armored Mages? Mystic monks? Lightning-slinging Rangers? Paladins that cast Fireballs that only affect Demons, Devils and Undead with a range of Line of sight? All possible.
And they can all be in the same campaign!
Now, if HERO is too complex to leave a good taste in your mouth, try Mutants & Masterminds. Its a neat fusion of HERO, D20, and a little Marvel Superhero thrown in for an RPG almost as flexible as HERO and almost as familiar as D20.
I know that some posters on this (and other) boards have started using it for their D&D campaigns, but I haven't done so myself.