I'd like to chime in a bit here. i'm a bit of a fan of HARP from Iron Crown. it's an addative percentile system (roll % and add in stats and skill mods) with a table look up (it's not chartmaster, there's a single unified chart, with some specifics for poisons and spells, then a few for combat based on weapon/spell type). the core book tells you how to design your own races and classes, the monster book tells you how to make monsters. so, supliment plus toolkit. Spells can be designed and modified on the fly as well. very fun, actually. most of the tables can be learned so you don't have to look them up, the combat tables are easy enough to reference. the strong part is the "on the fly modified" magic system.