Castles & Crusades Society member and FPQ Member (president? founder perhaps?), Duke Omote has created some kick ass house rules for Castles & Crusades, which he calls Advanced Castles & Crusades. One of the great features of C&C is that it is very easy to house rule the hell out of it. Omote demonstrates this with style with these rules, giving his take on elements of other editions of D&D. Want to add Feats ala D&D 3rd edition? Try his Talents rules. Like those Secondary Skills from 1st edition? Check out his rules for ‘em.
All of his stuff is great for supplementing your existing C&C game or perhaps easing the conversion from another version of D&D (e.g., 3.5 to C&C).
On his Rules Expansions page, he has rules for: