I was checking out The Same Universe wiki (a truly amazing wealth of material can be found there, including dozens and dozens of hex maps) when I came across this alternate spell casting system contributed by James.
It is a simple system for magical fatigue that increases the chance of spell failure the more spell levels cast. It bolts on to traditional D&D magic and allows a caster to cast any spell in her repertoire with a d6 roll + level vs Spell Fatigue, a new number to track (which increases with each casting, successful or not). A first level caster should be able to get several spells off successfully, but runs a chance of spell failure with unpredictable results.
I think it would go a long way toward "fixing" OD&D / B/X / AD&D style Vancian fire and forget magic for me. I have just never liked the feel of magic as presented in those editions. The house rule both increases the power of low level casters and makes magic work in a more intuitive way. At higher levels, it looks like it would limit casters a little bit with the chance of a spell failure, but high level casters are pretty dang powerful anyway.