This question is pretty simple: are there any feats for D&D 3.5 that allow a non-barbarian character to enter a rage mode? It doesn't matter if the feat is a third party resource. I also don't care about prerequisites.
I am after feats specifically, and not interested in non-feat options.
Best Answer
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)
Rage spell:
Doesn’t get any more direct than that.
Retribution Domain (Spell Compendium)
Thematically similar.
Strength Domain (Player’s Handbook)
Fluff is off, but the effect is pretty similar.
Wrath Domain (Spell Compendium)
Hatred Domain (Spell Compendium)
More like Favored Enemy than Rage, but eh, might work.
Destruction Domain (Player’s Handbook)
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.