HERO Grit! is a quasi-alternative to XP and also serve as an Action Point system. They are very appropriate for those games where the characters do not change from session to session as much, or at all (e.g. pulp, supers) and yet you still want a session based reward system. Then, at appropriate times in the campaign you can give them a pile of XP and take the game to the next level as it were.
HERO Sanity is a set of rules designed specifically for Cthluhu games, but I use them in my Narosia games as well since there is a horror element to that setting.
HERO Shotguns are an alternative approach to modeling shotguns in HERO System that I think is both more realistic and easier to play.
d20Hero Gritty Damage is an enhancement to the Mutants & Masterminds OGL d20HeroSRD system for grittier game play and campaigns where comic-book death is not realistic enough.