One of Mazinger Z's best attacks, the Rust Hurricane. Mazinger blows a jet of particle-charged wind from its mouth grill that corrodes the enemy into nothingness. Its successor Great Mazinger has the Great Typhoon, which is the same thing but without the rust effect, while Super PrototypeMazinkaiser has the Rust Tornado, an Up to Eleven version of Mazinger's attack.
Many Robeasts from Anime/Mazinger, Great Mazinger and UFO Robo Grendizer have breath-based attacks, usually expelling out fire, acid or even poison. One of the seven armies of Mykene Empire (the Reptilian Warbeasts) specialized in this kind of weapons.
Most beam attacks in Pokémon, such as Hyper Beam and Aurora Beam, work this way. Pokemon that lack a Breath Weapon have attacks that emerge from other body parts (get your mind out of the gutter; we're talking about hands, pincers, shell-mounted pressure cannons...)
Taken to the logical extreme with Recoome from the Ginyu Force, who gets shut down (literally) by Krillin upon attempting this on a wounded Vegeta, and blows up nearly all his teeth in the process.
In a technological variant, Sergeant Metallic (aka Major Metallitron) has a missile launcher in his mouth.
And in a particularly hilarious example, Super Buu uses a mouth beam to badly scorch Gotenks, and laughs at him. He's shut up rather quickly when Gotenks opens up wide and delivers the same attack back at him.
Ironically, the most horrifically dangerous breath weapon in the series is used by Dai-Kaio (the highest of all the Kais), described as one of the cheeriest and most benevolent beings to ever live. If hit by it, the recipient is immediately diced and then blown up. Supreme Kai notes that that attack would be a One-Hit Kill on anyone other than Buu, who can regenerate indefinitely. Beware the Nice Ones indeed...
Several of the Boomer robots from Bubblegum Crisis had beam cannons hidden in their mouths.
Mobile Suit Gundam had the Zeong. This attack was the one that finally did in the titular mecha, the Gundam.
The F91 and Crossbone Gundams have a heat vent in place of a "mouth", usually covered by the faceplate. It isn't really intended as a weapon, but if their "overheat" mode activates at point-blank range, the sudden blast of intense heat from the mouth has been shown to damage mobile suits.
The Menos Grande and some of the Arrancar of Bleach do this with their Cero attacks. Nnoitra has a slightly creepy variant: he fires his from the tip of his very long tongue. Ew.
Nelliel can eat Ceros and fires them back with the combined force of her own Cero, this technic is called Cero Doble.
There's also the well-known Burst Stream of Destruction / White Lightning from Blue-Eyes.
Slifer the Sky Dragon has two Breath Weapons, one for each mouth. His first mouth fires the yellow Thunder Force (the Japanese name is Chō den dō wa - Thunder Force/Super Electrical Conduction Wave - Thunder Force), aiming for attacking. His second mouth fires the blue Lightning Blast (the Japanese name is Shōraidan/Summon Thunder Bullet), aiming for reducing the ATK or DEF of the opponent's new summoned monster by 2000 points.
Piyoko has her Upchuck Bazooka attack to counter Di Gi Charat's Mekara Beam (which she shoots from her eyes).
Bellcross from Heroic Age does this several times, being sure to clean up the local space of annoying bug-aliens by using their own life force and energy to power it.
Some Zoanoids in Guyver have this; Derzerb of the Hyper-Zoanoid Five could breathe napalm, and the Zoalord De Galenos fires energy blasts from his mouth.
In the final episodes of Fullmetal Alchemist, a cyborg produces a number of guns from surprising places, including a tiny ball turret in his mouth.
From Potemayo, the things on the sides of Guchuko's head shoot a charring variety of beam.
The Tailed Beasts in Naruto are all capable of firing highly destructive balls of pure chakra from their mouths. Several of them also have other breath attacks, such as Shukaku's balls of concentrated air and the Two-Tailed Cat's fireballs.
Unlike the typical example of this trope, Breath Weapons are more commonly used by the heroes; Shizune can breath out poison mist, the Fifth Mizukage can spit acid mist and lava, the Sarutobi clan can all breath fire, Jiraiya and the toads can spit oil, etc. Which isn't to say that the antagonists don't get in on the action; for example, Danzo can fire wind bullets, while the Uchiha antagonists are all fire-breathers as well.
Parodied when Naruto tries to do this while (consciously) using his Tailed Beast's power. The 8-Tails describes it as sort of like vomiting, prompting Naruto to actually try and make himself vomit, which works about as well as you'd think (he wasn't in the right form anyway).
Many Digimon have breath weapons, at least until the higher evolutions when they usually grow actual cannons.
One Piece's Bartholomew Kuma had a mouth beam built into him to add on to his other powers. The Pacifista modeled after him likewise have a mouth beam built in them.
Franky on the other hand is equipped with both a flamethrower and built in nailgun.
A giant bird in the Skypiea arc had a flame-dial in its beak and both Buggy and Kidd had firebreathers in their crews.
Before any of these examples was Mr. 5. Using the Bomb-Bomb Fruit, he's able to make any part of his body explode, including his breath, which he loads into a gun.
In the first movie, the main antagonist ate the Goe Goe no Mi, which gave him this ability.
Zatch/Gash, in his namesake series Zatch Bell! / Konjiki no Gash!! gets a series of spells that short bursts of lightning out of his mouth, and in one of the more powerful spells, the lightning forms into a dragon. Not to mention he's also a demon, although he looks exactly like a human child hold the lines down his cheeks and the horn hidden in his hair.