Call of Duty is a good PDF. It has some pretty interesting PrCs, like the Archon, a law enforcer who focuses on the Lawful part, or the Sensate, a psionic Paladin. It has some options for swapping out Paladin abilities for other Paladin abilities, but Book of the Righteous does this much better. And there are some Paladin feats and other stuff, too.
Book of the Righteous is great, but it's expensive, and it's more general, focusing on a pantheon and divine characters, so there's less for Paladins. The Paladin is replaced by a more customizable Holy Warrior, but there are no PrCs for Holy Warriors in the book.
Of course, there's always Defenders of the Faith, by Wizards of the Coast. But again, this book is not particularly focused on Paladins, so there's some useful stuff in there, but lots of it is aimed more at Clerics.
Relics and Rituals has some nice Paladin spells, and some people say the spells are a bit powerful. On the other hand, many would say the Paladin spells in the Players Handbook are too weak.
FFG is coming out with Path of Faith sometime soon. Looking forward to that!