Termite soldiers will go out of their mound in the event of a breach to hold off the enemy (usually ants) while the workers rebuild. Sealing the soldiers out.
The Port Arthur Massacre had more than one sadly failed attempts. Nanette Patricia Mikac knelt down before a mass murderer to die whilst asking him, simply, to spare her children (unsuccessfully). Carolyn Loughton threw herself on top of her daughter, but while she survived, her daughter did not. Brigid Cook began an impromptu evacuation of the site and whilst shot, survived. Neville Quin was shot in the neck and survived trying to reach his wife.
Colonel Yuri Gagarin died on March 27, 1968 when the Mi G-15 he was piloting crashed near Moscow. He could have ejected safely from his failing craft, but chose to fly on past that point, because if he had ejected, the plane would have landed on a village killing everyone there.
Previously, Gagarin had tried to save his friend Vladimir Komarov's life by pulling one of these. The Soyuz 1 spacecraft was a poorly designed and completely doomed piece of junk that was only going to be launched due to pressure from the Politburo, and Gagarin knew it. Komarov was going to be the pilot, and Gagarin was his backup. Gagarin tried to use his clout to cancel the launch, and when that was unsuccessful, he tried to take Komarov's place. But the Politburo refused to allow a national hero on a flight they knew would not be two-way, and Komarov went up as planned. Almost every system on the craft began to fail almost immediately, and the mission was aborted. Unfortunately, the ship caught fire during re-entry and both the main parachute and reserve parachute failed to deploy properly, so Komarov hit the Earth without slowing down and was killed on impact.
Practically every posthumous recipient of the Medal of Honor or similar awards.
The living recipients generally qualify too. A lot them say that they fully expected to die.
Subverted, if the story behind the "Burghers of Calais" sculpture is to be believed: the six noblemen of the town who offered themselves in exchange for the town's safety ended up alive after it all, and the statue depicts them as not so much "heroic" as defeated and weary, but conscious of duty.
Flower, veteran matriarch of Meerkat Manor's Whiskers clan, embraced this trope when she went Mama Bear on a den-invading cobra, and got bitten on the face defending her pups.
Her son Shakespeare as well. First, he gets bitten on the jaw and thigh driving off a puff adder, then later, he has the misfortune of being the lone babysitter for his mother's pups when a rival pack comes calling. No-one's actually sure what happened, but the pups survived and Shakespeare was never seen again.
First Lieutenant John Robert Fox called fire on his own position on December 26 1944 in a successful attempt to stall a German advance. His body was located in a counter-attack the next day, along with those of around 100 German soldiers.
Captain Lawrence Oates, member of the Terra Nova Expedition (and expedition to the South Pole). While returning from the center of the South Pole with another three people (they were five but one died) his condition started to get worse due to frostbite and scurvy, slowing the others. He realized that due to him slowing them down the others wouldn't be able to survive. So, he asked them to leave him behind; when they refused, he said "I'm just going outside and may be some time", he put his boots and walked out of his tent into a -40cº blizzard.
The liquidators as well, soldiers and volunteers who took part in the cleanup operations to evacuate the nearby areas and to help stop the spread of the radiation. If it wasn't for them the disaster would have been far far worse.
NYPD Officer Arthur Kasprzak dies after saving seven family members from rising flood waters in his house after the devastation of Hurricane Sandy.
When the Halifax Explosion of 1917 was about to erupt, train dispatcher Vince Coleman had an opportunity to flee, but remembered that a train was due in town any minute. He stayed at his post and transmitted the following telegraph: "Stop trains. Munitions ship on fire. Approaching Pier 6. Goodbye." Coleman was killed at his post, while the trains involved stopped a safe distance away from the explosion, saving hundreds of lives. What's more, the message was communicated throughout the rail network, allowing other dispatchers to immediately respond, rerouting traffic and sending relief supplies.
Jalisa Granger, a young mother who died sheltering her baby during a tornado.
Don Lansaw jumps on top of his wife Bethany to protect her from a tornado and saves her life, but sadly dies in the process.
Compared to Chernobyl, the Fukushima 50 are (hopefully) an aversion. They are carefully being rotated to limit their total exposure to within international safety standards for emergency workers (with only a handful of cases where those have been marginally exceeded by accident), and thus far nobody is believed to have received any genuinely dangerous doses. The generators have now stablised, though cleanup is still needed.
In fairness, the Chernobyl disaster was, of course, far worse (massively higher levels of radiation, inferior or lacking safety equipment and procedures, much less transparency). Certainly a Crowning Moment of Awesome for the 50, but it likely won't turn out to be a Heroic Sacrifice.
Following the March 11, 2011 earthquake, Miki Endo, a young Crisis Management worker in Minami Sanriku near Sendai, broadcasted a tsunami warning and was credited with saving the lives of nearly 7,000 people in her town. In recently salvaged recordings from the broadcast, her co-workers can be heard pleading with her to evacuate as the tsunami approached. But she stayed at the mic, warning people to flee until the more-than-ten-meter wave crushed the building she was broadcasting from.
Fujio Koshita, at 57 the senior Otsuchi firefighter, died standing on top of the firehouse ringing the old warning bell, because the March 11th earthquake killed all electrical power in the town. His bell was heard ringing through the town until the tsunami swept him, and the firehouse, away. According to other firefighters, he violated his own rule about rescue workers; “Don’t die. Rescuers must stay alive” because your job is to help other people.
Arland Williams, the "sixth passenger" of Air Florida Flight 90. When the plane crashed in the icy Potomac River, he and five other survivors scrambled to the tail section of the destroyed aircraft. Twice he was tossed a lifeline from a helicopter. Twice he handed it off to others more severely injured than himself. When the chopper returned again he had slipped below the surface, the only victim to die by drowning.
The Four Chaplains on a crowded World War II troop ship. When it was torpedoed, the chaplains handed out life jackets to the men who made it onto the deck. When the life jackets ran out, they gave away their own.
Erwin Rommel, famed Nazi general, was condemned to death for his involvment in the July 20th plot against Adolf Hitler. The new 'blood guilt' (Sippenhaft) law meant that his family would have been punished as well. Rather than be executed as a traitor, Rommel accepted Hitler's offer of suicide and took a cyanide pill. His family was spared, and today Rommel is remembered for his civil conduct in warfare and willingness to help overthrow Hitler.
On July 27, 2008, a man opened fire with a shotgun at the Tennessee Valley Unitarian Universalist Church, because he disagreed with their beliefs. A church member named Greg McKendry deliberately took blasts from the shotgun to protect other church members, and died as a result.
On May 24 2008, Harry Potter actor Robert Knox saved his younger brother from a deranged knife wielder at a pub, losing his life in the process.
John Luther "Casey" Jones, a railroad engineer from Jackson, Tennessee, died in a train crash on a foggy, rainy night in 1900. He had ordered his friend, specifically, to "Jump, Sim, jump!". With his friend off of the train, Casey bravely controlled the engine to minimize impact, saving the lives of all of the passengers on board... except for Casey himself.
In a less known event of Casey's heroism, he personally climbed onto the cowcatcher of the train, while it was moving, to reach out and save a child who was immobilized by fear on the tracks.
Richard "Rick" Rescorla saved over 2500 people by leading them to safety during the 9-11 attack, and died in the process.
By extension, all of the firefighters and policemen who stayed in/near the World Trade Center in an attempt to evacuate as many people as possible.
John P. O'Neill, the FBI Officer who had led the investigation into Al-Quaeda after the USS Cole bombing and then got a job at the WTC went back into the building (after helping evacuate the daycare centre) to help anyone still alive. He was never seen again.
Michael Monsoor, US Navy SEAL; a grenade landed on a roof where he was part of an overlook. He jumped on it and sacrificed his life, taking the brunt of the blast and keeping the shrapnel from hitting any of his friends. He was posthumously awarded the Medal of Honor.
A mother in Springfield, Massachusetts, saved her young daughter by shielding her with her body while a tornado destroyed their home. 
Everyone's heard about the band on the Titanic, but fewer people remember her engineering crew. The ship's engineers remained at their posts until the end, keeping the engines running as long as they could so that the lights would stay on and stave off panic. None of them survived the disaster.
They quite possibly did more than keep the lights on. Questions have been raised about why the ship, unlike almost every other large vessel that's suffered damage on one side of the hull, didn't capsize. It's been suggested that the engineers kept working the trim tanks and pumps until all power was lost to keep the ship upright and allow the lifeboats to be launched from both sides. Since, as mentioned, none survived, we'll never know.
Let's not forget the people who gave up their own seats on the lifeboats. These people gave up their chance at survival to help strangers or to make sure that someone they loved who couldn't get a seat wouldn't die alone.
When the RMS Lusitania was torpedoed and sank in 1915, wealthy businessman Alfred Gwynne Vanderbilt gave up his life jacket to a woman with a baby, and was last seen buckling her into it. He did this knowing that he could not swim and would surely die.
Kirsty MacColl was on holiday in Mexico with her family, diving in an area that was supposed to be off limits to boats, when a speeding powerboat appeared heading straight for her son. She managed to push him out of the way in time for him to suffer only minor injuries, but she herself was struck and killed instantly.
Kansas City Chiefs running back Joe Delaney dove into a lake despite his own inexperince in swimming to save three drowning children. Sadly, not only did Delaney lose his life in the process, but only one of the three children survived.
Michael Patterson jumped into a creek to save a four year old girl who had fallen in. He saved the girl, but was paralyzed from the chest down from the injuries he sustained while saving her.
A couple of canine examples of this trope; George, a Jack Russell in New Zealand who was killed defending five kids from a pair of vicious pit bulls; and Chief, a pit bull in the Philippines who sustained a fatal bite protecting two women from a cobra.
Also this story about a dog who saved his suicidal owner's life by moving him off of some train tracks when his owner collapsed and passed out as a train was coming by. The owner was fortunate and quickly taken to the hospital but the dog sadly died when the train collided with him. This type of Undying Loyalty is exactly why dogs are properly referred to as "Man's Best Friend".
U.S. Confederate soldier Calvin Crozier, who was headed back to his home in Texas at the end of the war, got into a scuffle with some aggressive Union soldiers in the town of Newberry, South Carolina. During the fight, he non-fatally cut one of them in the back of the neck, but the soldiers later claimed he murdered one of them and set out to arrest and execute him. Crozier was long gone, but found out that an innocent man had been arrested in his place. He returned to the town and turned himself in to save the other man's life, and was unlawfully executed by the soldiers.
Saint Maximilian Maria Kolbe was a Polish Catholic priest who first was a missionary and thus went Walking the Earth for years. During World War II, this Badass Preacher was imprisoned in the Auschwitz concentration camp for openly speaking out against the Nazi regime through noth a newspaper and a private radio station, and sheltering (among others) 2,000 Jews. He willingly went into the hunger bunker in place of of another prisoner, a Polish woodworker who had a wife and kids; he continued to calmly celebrate Holy Mass in the cell for the other prisoners, and was finally killed by a fatal injection of phenol after all the other prisoners had already died of starvation. He was made a saint in October 1982 by Pope John Paul II, and the man whose life he saved was there.
Similar to the Casey Jones and Yuri Gagarin examples above, Gerald Stull stayed at the cockpit of his failing plane so that it would crash in the lake rather than the city of Monona, Wisconsin.
First grade schoolteacher Victoria Soto hid her students in her classroom's closet during the Sandy Hook Elementary School massacre when she was confronted by the shooter and she told him by lying that her students were in the gym. She then threw her body in front of her students and as a result she was killed.
It may be worth noting that, according to Wikipedia, Victoria Soto, along with five other employees, was posthumously awarded the Presidential Citizens Medal for "exemplary deeds or services performed for his or her country or fellow citizens." This award is the Second Highest civilian award in the United States, second only the Presidential Medal of Freedom and its military equivalent, the Medal of Honor.
Another Sandy Hook example: The school's principal died while charging the gunman in an attempt to stop him.
Spirit guitarist and frontman Randy California and his son got caught in a rip current while swimming in Hawaii in 1997. California pushed his son to safety, but was swept out to sea.
William David "Dave" Sanders. On April 20, 1999, as Eric Harris and Dylan Klebold opened fire on their classmates at Columbine High School, Sanders took charge and kept a level head as he directed hundreds of panicked students to safety. He then ran through the hallways, securing classrooms. He was shot by Dylan Klebold and died four hours later from blood loss.
Nicola Calipari, a high-ranking officer of the Italian Secret Services, in 2005. He had obtained the liberation of journalist Giuliana Sgrena from an Iraqi insurgent group and was taking her to Baghdad airport, when an American roadblock crew opened fire with on the Italians' car. By Sgrena's direct account, Calipari jumped in front of her and pinned her on the bottom of the car to shield her from the gunfire, dying in the process.
Gare de Lyon rail accident, June 27, 1988 in France, André Tanguy, a French Train driver, alerted his passengers about a runaway train their way and ordered to them escaped. Tanguy alerted the station about the problems until he was killed along with 55 other people, but Tanguy's sacrifice spared others. Daniel Saulin, who was driving the runaway train, failed to tell the station of his name or the train before hitting the alarm system, which would've guide the train to an empty platform. In hopes of the chances of a future disasters are reduced, the French Railway was overhaulled the driver's training and radio system was upgraded.
Louis Slotin became the second fatality the criticality accident to happen at Los Alamos, months after Harry K. Daghlian, Jr was killed from the same experiment with a plutonium core, which earned its demonic nickname. The Canadian born physicist and chemist was working for the United States’ Manhattan Project with a sphere type beryllium metal, as similar to the tungsten carbide Daghlian was working with when both accidently made contact with the core, creating a blue radioactive glow. Slotin quickly removed the beryllium with his hands and told the people in the same room as him to mark their location before going to get medical help for record keeping. Slotin received a fatal amount of radiation, which ultimately dying on May 30, 1946, but he recognized by the both his native Canada and the United States for his brave sacrifice and considered a hero for doing so.
Cleveland Elementary School principal, Burton Wragg, heard gunshots and both he and Mike Suchar, a custodian, ran to save as many children as they could. Both men lost their lives and 8 children were hurt in the shooting done by Brenda Ann Spencer, who was 16-years old at the time, but had they not got there, more lives would’ve been lost.