For a really simple set of fantasy rules, old school play style, Microlite20 is a pretty solid simplification of D20 3.5. The main rules fit on a page. There are full conversion lists for spells and monsters, and plenty of extra classes if desired. There are also optional spell casting systems, to use with or instead of the base system. Spell points, health points, Vancian casting, build a spell, runes its all in there. And tons of write ups for various game settings. FREE. 2000+ pages (repetitious) pages.