Killy, the protagonist from Blame, takes more physical punishment than any normal human would ever be able to survive. Most notably in one chapter, where he gets half his head blown off. It has its consequences, naturally, but he still gets better.
In one episode, Sailor Moon is thrown upside down into the air and hits the ground with her head without any lasting consequences. To be fair, all Sailor Senshi are regularly tossed around by the Monster of the Week and usually don't sport any visible injuries, but that example was particularly jarring.
Averted in Black Lagoon when Revy actually gets a concussion, rendering her non-responsive for about a whole episode, during her fight against Roberta. Granted, in the anime it took no less than a near-direct hit with a grenade launcher (just an armor-piercing round to the shoulder in the manga), but still...
She fully recovered less than an hour later, immediately after being in a car crash.
The Saiyans from Dragon Ball Z generally seem to have this, although Goku — despite the Easy Amnesia he suffered as a child — seems to be a notable case, and it was commented on multiple times by other characters, including fellow Saiyan Vegeta.
He did beat a guy in a combat robot suit by headbutting it.
Headbutting was practically kid Goku's finishing move.
Krillin: I hope he's got a seat belt in [the Saiyan Space Pod].
Yamcha: Why? To protect the ship from Goku's head?
Ryōga Hibiki of Ranma ½. Even for someone who uses Supernatural Martial Arts, he's considered insanely tough. His most well-know "technique" is the Bakusai Tenketsu, Training from Hell in which he slammed himself face-first into boulders so much that he toughened up way into the higher grades of Made of Iron. There was one gag where he headbutted a concrete pole so hard that he not only dented it (noting that it didn't hurt in the slightest), but it actually cracked in half and fell on his head with enough force to shatter into pieces after he turned around. This resulted in a Cranial Eruption and him commenting that it hurt a little. In at least the anime version of the "Waterproof Soap" story, after Ryōga's apparent "cure" has been revealed, Ranma casually lifts a huge boulder and slams him on the head with it, only for Ryōga to pop back up, completely oblivious, causing it to flip over and crush Ranma instead.
Possibly subverted in Wolf's Rain: Following the death of his lover, Hamona, Darcia violently and repeatedly smashes his head against the wall of his palace. He eventually goes insane, but not before seemingly recovering completely from the head trauma with no ill effects.
His insanity may instead be the result of Jagara's poison.
Appears in Tokyo Magnitude 8, where it is horrifically subverted. Yuuki is struck in the head by a piece of the Tokyo Tower and dies a few episodes (approximately a day) later from cerebral hemorrhage, showing increasing amounts of weakness and nausea after the impact.
In an episode of Sayonara, Zetsubou-Sensei, the girls go to break Sensei out of government confinement using their respective skills. The Apologizes a Lot girl, Ai Kaga bangs her head on a security person every time she bows to apologize, and kills tons of people through the hardness of her head.
Naruto gets his head slammed into the ground a few times in the series. Most of the time, his Kyuubi state reacts to the trauma.
Pain. In Naruto Shippuden 167, he was punched in the face, hammered into the ground by a 5-10 ton boulder, and thrown with incredible force head-first into a cliff, courtesy of 6-tailed Naruto. And he didn't get even a scratch. Considering he's practically undead, that makes sense of why he didn't die.
The fairly strong Jugo punched Killer Bee right in the face (and followed it up with a rocket boosting, but Bee appeared to have dodged that somehow), which doesn't slow him down at all.
Naruto tries to exploit this much later. When he catches up with Tobi during the 4th Ninja War arc, Naruto immediately headbutts him. Unfortunately, Naruto discovers that Tobi has an even harder head.
Sakuragi from Slam Dunk has a really hard head. His most well-known fighting move is his headbutt. It doesn't even hurt him when he headbutts the floor of a basketball court with all his might.
Black*Star from Soul Eater once fell from the sky and landed head-first in a crater and comes out unscathed.
While it is never officially stated, in Saint Seiya the eponymous Seiya is hit in his head very often. And by "very often" we mean every single time there's an unstoppable, all-powerful technique used on Seiya, it'll unerringly hit him in the head. And in fact he usually seems to be more affected by attacks in his body than attacks in his thick skull.
In Fullmetal Alchemist, Winry hits Ed on the head with a wrench, often, and he is usually shown collapsed on the ground, bleeding more than enough to be worrying. Strangely, always Played for Laughs, since he inevitably recovers.
The titular InuYasha tends to get thrown through the terrain face-first a lot. Usually, the terrain gives up. In fact, he never responds to head injures, and the only damage he pays attention to is the kind thatmakes it harder to swing his BFS around.
In Natsuiro Kiseki, Natsumi hurls a wooden bucket at Koharu's head, since she thought Koharu was a boy peeping on her and her friends in the bath. This renders Koharu unconscious for several minutes, during which nobody helps her, not even her own twin sister Chiharu. When Koharu finally wakes up she proceeds as normal with only a mild headache.
Horribly, terrible subverted in Angel Beats!. A "simple" blow to the head from a bottle flung by her drunk father causes a character to collapse a few days later from intracranial bleeding, wake up for a couple hours with complete aphasia, and then die.
Akane from Urayasu Tekkin Kazoku. She still feels agonizing pain when something falls or thrown at her, but if at another person, it would leave a lumping bruise or unconsciousness. In one chapter when several kids were marooned on a small island, her head was used to crack open coconuts.