I know there are feats for maximizing/heightening/quickening or otherwise enhancing spell-like abilities, but are there equivalent ones for supernatural abilities? Specifically I'm looking for things that could work for a Dragonfire Adept's breath weapon, which is listed as a supernatural ability.
Tome of Magic has Empower Supernatural Ability, Enlarge Supernatural Ability, Extend Supernatural Ability, and Widen Supernatural Ability. They have the effects of the metamagic feats of the same name, except that they work on Su abilities, once per day per time you take the feat.
Races of the Dragon also has a handful of breath feats that work for dragonfire adepts. Unlike metabreath feats, which require a breath weapon that has a cooldown “expressed in rounds,” breath feats merely require that you have and use a breath weapon to activate them. Entangling Exhalation is by far the best of these feats; the others are Exhaled Barrier, Exhaled Immunity, Extra Exhalation, and Furious Inhalation.
Finally, note that while Draconomicon’s metabreath feat requirement is having a breath weapon with a cooldown expressed in rounds, their benefit can apply to any breath weapon. Thus, if a dragonfire adept has some other breath weapon (from the dragonborn template from Races of the Dragon, perhaps), he qualifies for metabreath feats, which may be applied to his dragonfire adept breath weapon. This would add a cooldown to the dragonfire adept’s breath weapon when used this way, however. Plus, ultimately, few of the metabreath feats are all that impressive; I probably wouldn’t bother with the hoops you have to jump through to do this.
I strongly recommend Entangling Exhalation for all dragonfire adepts. The rest of these feats are fairly mediocre.