Alfred Pennyworth became this as the series progressed. I mean, come on, the guy was a freakin' former S.A.S. soldier! And he has helped Bruce solve cases, invent new tech, heal his (both personal and Bruce's) own wounds, and managed to fight off thugs that would typically be more physically fit than him, as well as carrying extremely powerful firearms, with his bare hands!! Though, he wishes to remain a harmless butler.
The fight between Namor and Aquaman in DC vs. Marvel, as seen in the Trope Image.
Rorschach from Watchmen: He uses several household items to give himself a chance to get away. To wit, he improvises a flamethrower with hairspray and a match as he flees upstairs, on the basis that people are reluctant to chase a psycho up a burning staircase. The first person up the stairs after him gets a handful of black pepper thrown directly into his eyes and his floundering gives Rorschach the time to draw his last weapon—his compressed-air-powered grappling-hook gun,
In the video game Watchmen: The End Is Nigh, you can control Rorschach in battle. He's not nearly as skilled a fighter as Nite Owl (who uses an adapted form of martial arts), but he makes up for it with absolutely devastating strikes, and freely uses weapons, such as nightsticks, bottles, knives and crowbars, which Nite Owl refuses to do.
Ozymandias fights dirty too. It's not immediately obvious, but he's perfectly willing to take advantage of any psychological weaknesses his opponent has and uses the environment for his own benefit. Of course, with him, what looks like an Improvised Weapon was probably specifically placed right there hours ago.
During his final confrontation with Rorschach he pulls his mask across his face to gain advantage. And people keep on harping about Dollar Bill's cape...
Nightwing, despite being a Technical Pacifist, does this with acrobatic flair. He basically fights like Batman with a sense of humor: nose tweaks, groin kicks, and distractions in the form of ass smacks are not outside of his domain.
When you're on your own, behind enemy lines, no artillery, no air strikes, no hope of an evac, you don't fight dirty. You do things that make dirty look good.
Vick "The Rain" from 100 Bullets is not above fighting dirty and will even use his own allies as human shields.
Much of the G.I. Joe comic books involves this trope. Around issue #75, Tunnel-Rat emerges from a well, tunnel and mows down about ten Cobra soldiers from behind. An issue of Special Missions has one Joe save another by simply beating the hell out of a captured CIA prisoner for needed intelligence.
Despite being better known to the casual fan for his ninja ways, never forget that Snake-Eyes is an Army Ranger, and is more than happy to go for the grenades or machine gun as a ninja armed with melee weapons advances.
Cerebus, being a veteran mercenary, knows better than to fight fair. Early in the series, he was face-to-face with a rebel mastermind and as his opponent strode out onto a bridge, finally revealing his identity, Cerebus heaved a rock at his head, causing him to stumble off the bridge to his death. Afterward, this exchange took place:
Lord Julius:That wasn't exactly fair, was it? I mean, he thought you were going to fight to the death with swords! Cerebus:He is dead and Cerebus is alive... You can't get much fairer than that.
Throughout his adventures, Corto Maltese does his share of kicking people in the nuts and shooting them in the back.
In the final issue of Tim Drake's Robin series, he's forced to fight Lady Shiva. Tim knows that he's hugely outmatched, so he poisons her food two days before the fight with a heart-rate dependent neurotoxin.
The following exchange from a comic where Deadpool and one of the forgettable 90s X-Teams are facing a villain who can dampen mutant abilities:
Mutant Super Hero: All right, just because our powers don't work, doesn't mean we're helpless. We should engage him one-on one in hand to hand combat with our strongest fighters going first until he drops.
Desolation Jones has the titular character lampshade his status as as a combat pragmatist. When attacked by a fearsome S&M clad bodyguard, Jones explains that the combatant who wins isn't the strongest but the one who cares the least for holding anything back. The sickly old man then stabs his finger into the charging guard's eye socket and pulls him skull first into a wall, knocking him unconscious. He also makes liberal use of the Groin Attack.
John Constantine the Hellblazer is a dirty fighter, no matter how much he denies it. He's strong enough to fight a bar full of hooligans, but will keep in mind anything his hands can reach, such as bottles and chairs. Heck, he even once used a handful of wheat grain...yes, the thing in your cereals, to knockout a robber.
Iron First once went up against the Wrecking Crew, a team of superpowered baddies who each have the power of an Asgardian god. Iron Fist, when he's not using his trademark superpower, is basically just a highly skilled martial artist. The following exchange took place:
Piledriver: You is just one super-punk and you're a skinny one at that! How you gonna hurt me?
Iron Fist: There are ways. (Slams his hands over Piledriver's ears. Piledriver collapses in pain)
Thunderball: Smart move, hero. That was probably the only way you could have hurt him.
Parodied in Astérix in Britain. The Romans, observing that the... British take a regular break every afternoon to have tea, decide to attack at tea-time.
Bullseye once used his own tooth to kill a man. The tooth was spitted from his mouth and went through the guy's skull. He's even considering using his own feces as a weapon. Ugh!
Moon Knight is more than happy to use truncheons, knives and a spiked steel cestus in the fight for justice.
Spider-Man has been known to fight this way, using his webs to blind/restrain his enemies as well as finding all sorts of ways to humiliate them, specifically so they'll get angry and make mistakes. In the Ultimate series, he even gave Ox a wedgie. Having been bullied in school, Spidey was of course, very ashamed of himself.
Nikolai Dante is frequently described as the dirtiest fighter in the empire. For example, Nikolai (a Badass Normal at this point) fights Konstantin Romanov, a man with powers that effectively make him a walking nuke, for the right to be Tsar. At the beginning of the fight, Konstantin mentions that he should have shot him when he had the chance. Instead, Nikolai waits for the custom virus he released into the arena to take hold, disabling Konstantin's fusion powers. He taunts and dodges powerless Konstantin for the next few minutes, and only shoots him just before his powers come back. Dirtiest fighter, indeed.
Marv from Sin City is physically capable of handling most opponents in battle but often uses whatever he can get his hands on simply because he's that damn crazy. Case in point: using a nearby hatchet to shove into a cop's groin.
Lusiphur, the protagonist of Poison Elves, almost never fights cleanly if he can help it. He isn't above such tricks as pretending to beg for mercy in order to get the jump on a superior opponent.
2011 DC Universe reboot: Hal Jordan starts boasting to Batman that with his ring, he can easily take down Superman. By the time Batman blinks, Superman has Flash Stepped up to Hal and punched him out.
In ElfQuest when the Wolfriders are attacked without warning by Guttlekraw's trolls:
There is no fairness, no grace, no nobility in the Wolfriders' method of combat. They obey but one rule: survive by any means, no matter how cruel or bloody.
Anathos from Les Légendaires, despite being more than capable to defeat practically everyone without cheating (he once stopped Jadina's punch with one finger), but will gladly make use of this trope if it makes things more practical, his duel with Darkhell's and Elysio's fusion being the most egregious example:
Darkhell / Elysio: It seems our forces still are equal in a fair fight.
Anathos: Who talked about a fair fight? (backstabs him with his flying sword)
Surprisingly subverted later in the story, where, after finding out the Legendaries are trying to ambush him, he intends to have his minions confront them rather than go fighting himself, but Calende shows up and advices him not to do so, commenting that it will result in him being remembered as a coward. As a result, he follows her advice and go to fight them in person with his Hellions.
Many of Marvel's heavy hitters such as The Hulk, The Thing, etc. will use whatever is available when goaded into a fight.
These heavy hitters sometimes also have to use their strength in creative ways on those rare occasions when they're facing someone with even more raw power than them. In the Future Imperfect one-shot The Incredible Hulk was outclassed by the Maestro, his Evil Counterpart. The good Hulk evened the odds first by hitting the Maestro with a blatant Groin Attack, then by throwing Captain America's shield into his chest (the shield is unbreakable, but Cap wasn't strong enough to throw it hard enough to injure the Maestro) and eventually using Doctor Doom's time machine to transport the Maestro to ground zero of the same gamma explosion that gave Bruce Banner his powers, vaporizing the Maestro on the spot.
Despite having dangerous mutant powers and being trained extensively as a martial artist and staff fighter, Gambit will often cheat in his fights anyway.
The Mark Shaw version of Manhunter preferred to avoid even the fighting part. In his first comic, he ambushed Dr Alchemy after taking advantage of his fondness of redheads with 'balconies you could do Shakespeare off of'. Another time, he waited till The Penguin was in the bath.
The Fables, in the series of that name, are often quite pragmatic. In the first issue, Bluebeard tells Cinderella, during a fencing lesson, that "we train with real swords for real battles." And when Geppetto challenges them, they are quite willing to use modern technology combined with magic to defend Fabletown from his forces, and then to turn the battle against him. Finally, when Snow White calls Prince Brandish out for a sword duel, she takes a moment to kick off her high-heeled shoes, opting to fight in bare feet, unlike many women in comics.
Lady Shiva goes on about how the Blackhawk Squadron will never even realize she's cut them up and butchered them by the time she's done with them, when they decide to just shoot Shiva as she's gloating.
When Doc Fate meets his counterpart Felix Faust, Faust attempts to pull a magical duel. Fate just kicks him in his balls.
Superman can be like this when the situation allows. For instance, a Post-Crisis story had him facing a dire threat and Metallo was standing in his way for a fight. Superman contemptuously responds he has no time to deal with the cyborg twerp and simply blasts limbs off with his heat vision until he is a pile of limbs and moves on. He often notes that he holds back a lot and tries to fight 'fair' in order to provide a good example, but in a tough situation will start to use his powers in many, many creative and nasty ways.
In the beginning of the story The Return of Bruce Wayne, Bruce Wayne gets stranded in the time of the cavemen and is eventually captured by a tribe led by Vandal Savage. They intend to eat him, but wanting some entertainment, Vandal challenges Bruce to a duel, but cheats by first ordering his men to cut Bruce several times and then leave him tied up on the muddy ground overnight, reasoning that even if he survives, the blood loss, hunger, and infections will leave him too weak to fight (the tribe allows this because they want Bruce to lose, and praise Vandal for being smart). Fortunately, a young boy (who wears face paint that resembles Robin's mask) from a rival tribe frees Bruce and returns his utility belt, which happens to have antibiotics. While escaping, Bruce is able to defeat the tribesmen and Savage himself.
While still a skilled martial artist, Huntress is nowhere near top-tier, and supplements her fighting style with these sorts of tactics, from improvised weapons to crotch shots. If the situation is dire enough, she'll just fill her opponent with crossbow bolts rather than get into hand-to-hand combat.
Deadshot was once challenged to a duel by a fellow assassin, who established that they would draw on the count of three. Deadshot shoots her dead right after she says "one".