I've been buying the Warhammer game supplements from Green Ronin (haven't played it yet) but that spell system (and the whole game actually) is like a blend of D&D and Cthulhu. The spell system is pretty dark, and you have to roll certain # of d10's to hit a target # to get a spell off. This is modified by your magic score, (how many d10's to roll) and if you have the right spell ingredients you get a boost. The twist comes if you roll 1's...or multiples of the same #, then the spell can backfire and really, really bad things can happen to you. There's no limit on how many spells you can cast, no Fatigue or anything, you just run the risk from as small as having your eyes flash in another color, to full blown possession by an Archdevil that eats your soul. Depending, of course, on the magnitude of power you try to cast and botch.
It's a neat system.