Sword and Crown Book

The martial arts rules are a lot of fun, but you'll want to be careful lest you end up mucking up the game too much with them. I think I prefer the original Oriental Adventures ninja, though, for some better abilities and a bunch of multiclass-induced flavour.

This book also contains some of the weirdest and most ridiculous nonweapon proficiencies. As good argument as any to not bother with the whole subsystem at all.