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.
TL;DR: Druids and sorcerers have access to feats that offer Rage-like effects, while the Touchstone and Planar Touchstone feats offer access to a smorgasbord of effects, including Rage-like effects.
The best of them is the Catalogues of Enlightenment Planar Touchstone, which gives the granted ability of any cleric domain – the Passion Domain gives a 1/day self-only rage spell, which is explicitly “otherwise identical to a barbarian’s Rage.” This is the only feat option I could find that might qualify as Rage for the purposes of prerequisites.
White Dragon Lineage [Draconic]
Dragon Magic has a number of feats for sorcerers to explore the draconic origins of their magics. For 3rd-level sorcerers who have taken Draconic Heritage (white), White Dragon Lineage is a feat that allows you to expend an arcane spell slot to get temporary HP and resistance to cold equal to 5× the spell level, lasting for 1 + Cha rounds. It’s explicitly like a barbarian’s Rage but is not Rage for prerequisites.
Wolverine’s Rage [Wild]
This feat from Complete Divine allows a druid to expend a daily use of Wild Shape to get +2 Str, +2 Con, and −2 AC for 5 rounds after he or she has been hurt. Again, this is not true Rage for the purpose of prerequisites.
Touchstones
Sandstorm has a feat, Touchstone, and Planar Handbook has a related feat, Planar Touchstone, that allow you to get one of many different powers by visiting certain historic or magical locations. These two feats are incredibly versatile, and can get you a lot of stuff.
Catalogues of Enlightenment (Planar Touchstone)
A planar touchstone option, the Catalogues of Englightenment have as its base option “Choose a cleric domain; you gain the granted power of that domain.” There are a lot of cleric domains, and several of them have options similar to Rage. The problem is, the Catalogues don’t specify what to treat your cleric level as if you aren’t actually a cleric. The most reasonable answer is to use your character level, but square that with your DM. Note that the Catalogues of Enlightenment are considered an 8th-level encounter, so you may not be able to put this in your backstory.
Passion Domain (Eberron Campaign Setting)
For a total time per day of 1 round per cleric level, as a free action you can act as if under the effect of the rage spell. This granted power is a supernatural ability.
Rage spell:
The effect is otherwise identical with a barbarian’s rage
Doesn’t get any more direct than that.
Retribution Domain (Spell Compendium)
Once per day, if you have been harmed by someone in combat, you can make a strike of vengeance with a melee or ranged weapon against that foe on your next action. If this strike hits, you deal maximum damage.
Thematically similar.
Strength Domain (Player’s Handbook)
You can perform a feat of strength as a supernatural ability. You gain an enhancement bonus to Strength equal to your cleric level. Activating the power is a free action, the power lasts one round, and it is usable once per day.
Fluff is off, but the effect is pretty similar.
Wrath Domain (Spell Compendium)
Once per day, you can subtract a number of points from your Wisdom score equal to or less than your cleric level. For every 2 points you subtract from your Wisdom score, add 1 point and add them to your Strength score. You suffer all the effects of reduced Wisdom, including access to spells and bonus spells, reduction of Will saves, and penalties on Wisdom-based skills. This trade between ability scores lasts for 1 round per cleric level and cannot be ended prematurely.
Hatred Domain (Spell Compendium)
Once per day as a free action, choose one opponent. Against that foe you gain a +2 profane bonus on attack rolls, saving throws, and Armor Class for 1 minute.
More like Favored Enemy than Rage, but eh, might work.
Destruction Domain (Player’s Handbook)
You gain the smite power, the supernatural ability to make a single melee attack with a +4 bonus on attack rolls and a bonus on damage rolls equal to your cleric level (if you hit). You must declare the smite before making the attack. The ability is usable once per day.
Possibly more like Smite Evil, but again, might work.
Sunken City of Pazar (Touchstone)
For beating this 4th-level encounter, you get Powerful Build 1/day for a minute. The higher-order version makes it last for 8 hours. Powerful Build makes you behave in many ways as if you were larger, which replicates a lot of the functionality of Rage.
Pyramid of Amun-Re (Touchstone)
6th-level encounter, you get temporary hit points 1/day for an hour (24 hours as the higher-order version). Still, seeing as it requires a Concentration check, not that similar to Rage in fluff though the concept of temporary hit points is very much in keeping.
The Life Molds of Neumannus (Planar Touchstone)
The fluff is completely different, but the higher-order ability (i.e. needs recharging) of this planar touchstone (a 12th-level encounter) is an ability to temporary gain a bonus to strength, as well as DR 5/adamantine, resistance to fire and acid, and a penalty to Dexterity by becoming more construct-like.
Densahl’s Challenge (Planar Touchstone)
The higher-order ability of this 13th-level encounter is a temporary damage boost equal to your character level. You get to choose to use it after learning whether or not you hit, which is nice, though the damage is low for something you get so few uses of.
Best Answer
Polearm Master
The Polearm Master feat gives spears some nice features:
So you get a way to use your bonus action for another attack, and you can take opportunity attacks when enemies enter your reach, instead of just when they try to leave.
It is worth noting that spears were added to the Polearm Master via an errata to the Player's Handbook, so early editions may not contain spears as eligible weapons for the feat.
Combining with the Sentinel Feat is one of the strongest available combos.
The first bullet of the Sentinel feat says:
Combined with Polearm Master, you can stop creatures from moving for the rest of the turn when they enter or leave your reach, depending on when you take the opportunity attack.
This combo really shines when combined with Reach polearms, the lance, glaive, halberd, and pike. With Reach, Polearm Master and Sentinel allow you to stop approaching enemies before they can even reach you. You are able to stop them 10 feet away from you, beyond the reach of many enemies.
Unfortunately, spears lack the Reach property, but the lance is a Reach weapon, and more importantly, it does not have the Heavy property, though it does require two hands to wield unless mounted.
Unearthed Arcana: Spear Mastery
There was a feat called Spear Mastery included in a 2016 Unearthed Arcana article. It has been passed over in two significant splatbooks since then, Xanathar's Guide and Tasha's Cauldron, so it seems quite unlikely it will ever see print in an official book. Nevertheless, it can still be found in this UA Article, and I will reproduce it here for convenience:
In the article, Mike Mearls goes on to give some personal commentary on why he likes the feat:
As always, Unearthed Arcana material can be used only at the DM's discretion.