Inspired by Uncharted: Drake's Fortune, The Wrath of El Dorado lets you toss out a gold statue that releases damaging ghosts. It's an instant fix for clearing out hunkered down opponents.
The Cintamani Stone from Uncharted 2: Among Thieves works like a healing grenade to quickly revive downed teammates. Doesn't seem to turn them into mindless blue brutes, either.
The Spirit of the Djinn from Uncharted 3: Drake's Deception gives you a speed boost, and even lets you become invisible while rolling or jumping off a rope swing. A perfect way to close the gap for melee kills.
The Staff of Ayar Manco lets you see all your enemies within a certain radius, though they can destroy it once they find it. It's from Uncharted: The Fourth Labyrinth (a standalone novel which I'd admittedly never heard of before this point).
Indra's Eternity (inspired by the Eye of Indra motion comic) lets you trap enemies in a sphere that both slows them down and pens them in. They're easier to clean up that way, but they're still dangerous.
The Savior will accompany you in battle, reviving you and your teammates when you're downed or distributing ammo when you run low. Just don't expect him to get you many kills.
The Hunter chases down the nearest opponent and puts them in a chokehold, leaving them vulnerable for a one-hit kill. Make sure you seal the deal before they break free.
The Sniper defends a set position with a high-powered rifle, though enemies will be able to see her laser sight. Fortunately, you can reposition a Sniper without having to buy her again.
The Brute dishes out and soak up a ton of damage, but he's slow. Deploy the Brute and his minigun wisely and change the course of a match, deploy him unwisely and your opponents will get a big cash bonus for taking him out.