Monte Cook's Arcana Evolved: A Variant Player's Handbook (edició 2005)

This is a d20 game system that can be played alongside Dungeons and Dragons. I've never played in a Arcana Evolved only campaign, but even if you never do there are a lot of interesting classes, spells, feats, races to add to your generic D&D game. The layout of the book and the illustrations in Arcana Evolved are far better than their counterparts in the D&D core rulebooks.