Does anyone know of a supplement for the mechanics of magic in D&D 3.5? I was thinking of something that lets a caster cast more than his daily allotment of spells, but at a penalty of inflicted subdual damage or something. The reason I want to do this is for flavor and partly to complement my explanation for magic. For example, let's say a caster has used up all of his or her spells for the day, but suddenly the party desperately requires the use of a spell. I suppose the caster could say, "Jeez...crap guys, sorry but I'm exhausted, I just don't think I can do anything for you," and let heroism flicker away. Boring! Suppsing that magic isn't supplied by some exerior entity or doesn't come from components, but is something inborn, intuitive and dependent on endurance of the mind, I think it'd be cooler if the caster, despite having pushed it to the bone for the day, mustered up strength and cast whatever spell the party needed, but at physical detriment reflective of the stress on the mind, with varying levels of pain dependent on the level of spell being "over-cast." I'm aware that this might resemble Mindcraft somehow (don't want to give the idea of creative theft from a Mr. Doug Meerschaert, planesdragon), but I've actually been mulling this idea of magic around in my head for a while, but just haven't taken the initiative to actually implement it.
Advice is welcome, since I might have this whole thing wrong.