The Ur-Example, he has the power to - you guessed it! - eat and digest any form of matter. This not a joke. The supervillains he ate know very well.Everyone on his planet has this ability, supposedly due to natural selection after other edible things were poisoned by microbes.
M.E. Lad has eaten his way out of at least one cage. He is also one of the few characters possessing this ability to demonstrate the logical but downplayed corollary power of Acid Spit. Think of the "Alien" movies and their 'molecular acid for blood', but a controllable projectile. In a rarer example, Concussive Belching: two attackers tried to drown him in a grain silo. He ate all the grain, and the lock on the door, and then let out a belch powerful enough to knock the two men off their feet.
He can eat laser beams, on the grounds that energy is also a form of matter. He's also eaten an explosive device nobody could destroy, remove, or molecularly alter. People were reminded why they kept him around: as a plot device. (Sadly, he was driven insane in the process.)
Similar to M.E. Lad is Upchuck, the alien "Gourmand" of Ben 10's transformations. Only he can't eat anything that's edible to regular humans.
In Phil Foglio's adaptation of the Myth Adventures series, dimensional-castaway and powerless-magic-instructor Aahz will occasionally snack on various nearby objects, which have included a goblet and a doormat. The behavior was not specifically mentioned in the source novels. However, the cuisine from Aahz's home dimension of Perv (making him a P-e-r-V-E-C-T, thank you very much) is as disgusting to the reader's sensibilities as Asprin could make it, which may mean that Foglio's visual joke is a Justified Trope. However, as Aahz is only depicted doing this during long segments of exposition, it may simply be Foglio poking fun at Asprin's writing depicting Aahz chewing the scenery.
The comic book Chew features a man who has the power to psychically see the lives of anything he eats. He's a homicide detective, and is often forced to use his power to solve cases. Unfortunately, he can't turn the power off. He eats a lot of beets, because it's the only food he can eat without having to watch its entire history.
Grossout from Scare Tactics was known to eat small dogs when hungry, and once ate a pillow in his sleep.
The godship's spawn of Supergirl story arc Bizarrogirl are able to eat anything and everything: organic lifeforms, rocks... Then they return to their progenitor planet-eater where they're digested. Supergirl calls them "omniphagus suplicants".
Bizarro: Bugs came from godship and started eating. They ate Bizarros, buildings, even part of Bizarro World itself. Once they am full, they fly back into godship and more would come out. Supergirl: Huh. Omniphagus supplicants to produce fuel. Interesting Bizarro: Huh? Supergirl: Those things go out and eath something, then they return to the ship and are processed.
Galactus. It was bad enough when he ate planetary energy alone. After merging with the Gah Lak Tus swarm from Ultimate Marvel, he also gained their abilities and (and dinner palate) as well. With the swarm, Galactus could eat anything and everything on a planet, including people and objects. And you're not even safe at a subatomic level, since the swarms can shrink themselves as small as necessary to get their meal.
Also his daughter Gali/Galacta; she lives on Earth, tries to leave the locals in peace, and eats alien invaders and bioweapons. It takes all her willpower to NOT eat the yummy Earth. And it has gotten even worse since she became pregnant.
Transformers: Unicron would like to point out Galactus is not the only planetvore around.