Uses for simply having Rage?

[Jowgen]

One of my characters will be getting rage 1/day from a PrC, and it's completely wasted on the character and I can't trade it for an ACF, so yeah. Is there any benefit that can be gained purely by having rage?

Like, items that only work for creatures that have rage?

[Telonius]

Channeled Rage, a feat from Races of Destiny, allows you to spend a Rage to add your Strength bonus to a Will save. You have to be a Half-Orc, though.

Unfortunately I don't know of any items that require you to have Rage as a class feature. You could probably argue that it merits a 30% price deduction if you're crafting it (similar to requiring a class or alignment to use), but as far as I know there's nothing official about that.

[Flickerdart]

There's an item in the MIC that lets you spend Rages to heal yourself, but I don't remember the specifics.

[Oddman80]

Pathfinder has the weapon enhancement "Furious" that treats the weapon as being +2 better than it normally is if the weilder is raging.

[jiriku]

Berserker, +1 equivalent enhancement, your weapon deals +1d8 damage while you are raging.
Brash, +1 equivalent enhancement, while raging you are immune to fear, and your rage duration gains +3 rounds
Whirling frenzy rage variant grants +2 dodge bonus to AC and Reflex saves -- a useful defensive perk no matter what you're doing with your actions.

[ericgrau]

There's an item in the MIC that lets you spend Rages to heal yourself, but I don't remember the specifics.
cloak of predatory vigor (1400 gp): 2/day while raging swift action to heal your HD in HP
maks of blood (3,300 gp): +1 round to rage, spit 4d6 acid damage 2/day as a standard action.

[DrMotives]

Is there any chance you're a gnoll? Races of the wild has a feat for gnolls that gives them a bite attack, rage is a prereq for the feat. It explicitly says they don't need to enter rage to use the bite, just have the class feature on their char sheet.

[Thurbane]

Several items in the MIC only function for those with Rage:

Berserker (weapon enhancement)
Brash (weapon enhancement)
Cloak of Predatory Vigor
Mask of Blood