Memorial Day is the unofficial start of summer in the United States, and it's a day to remember the contributions of all veterans who have served in the armed forces.

(Actually, there's a little untruth in that statement, just to see if you're paying attention. Veteran's Day, in November, is the day on which we celebrate all veterans; Memorial Day is dedicated to those who made the ultimate sacrifice for their country.)

Recently, the U.S. Marine Corps awarded the Navy Cross, the nation's second-highest medal for valor, to the family of Sgt. Rafael Peralta, who was killed in Iraq in 2004. According to Marines who were with him at the time of his death, Peralta was shot in the head, but managed to pull a grenade that the enemy had thrown under his body, saving the lives of the others who were with him.

That's a sad and somber story, but it's also a heroic one. Over the nearly 15 years since the attacks of 9/11, we've had many such stories, and of course there are countless others from earlier conflicts. We can't possibly list them all here--from Navy SEAL Lt. Mike Murphy who sacrificed his own life to try to save his team in Afghanistan in 2005, to Lt. John R. Fox, who in World War II called in artillery fire on his own position, knowing it was the only way to stop a German advance that threatened to wipe out his unit. (Both men were posthumously awarded the Medal of Honor; in fact, Fox was one of seven African-American soldiers who were so honored many years after the war, in 1997.)

As we all take a break from our important work this weekend, here are a few more fitting examples of courage and honor to inspire you.

The protectors

The idea of literally diving on a grenade, as Peralta's fellow Marines say he did, has happened enough times that it's almost shorthand for any heroic self-sacrifice. Among the documented cases of others who have done this to save their comrades since 9/11 are Jason Dunham, a Marine corporal who covered a grenade with his helmet to save other Marines at the loss of his own life in 2004; Michael Monsoor, a Navy SEAL who smothered a grenade on a rooftop; and Ross A. McGinnis, an Army specialist who dove on a grenade that had been thrown into his humvee in Iraq in 2006, sacrificing himself to save four others in the vehicle. All three men were posthumously awarded the Medal of Honor.

The four chaplains

At the start of World War II, the United States drafted all kinds of professionals into the military, including rabbis, ministers, and priests to serve as chaplains. Four of these newly commissioned chaplains were heading together to England to join their units, when their ship, the SS Dorchester, was torpedoed and sunk by a German submarine. The four clergymen--a rabbi, a Catholic priest, and two Protestant ministers--gave their life jackets to other troops when the supply ran out, and all four perished as a result.

The volunteers

In what might look now like a prelude to the War on Terror, U.S. Army Rangers were involved in a fierce, multi-day firefight in Mogadishu, Somalia in 1993, after insurgents shot down two American helicopters. One of the pilots survived but was trapped, and a pair of special operations soldiers from the Army's Delta Force volunteered--insisted, really--to land on the ground and protect the pilot from thousands of insurgents, despite almost certain odds of death. The battle, which was portrayed in the 2001 movie Black Hawk Down, resulted in both men's deaths, although the pilot survived. Both men, Randall David "Randy" Shughart and Gary Ivan Gordon, were posthumously awarded the Medal of Honor.

The Marine who went back

John Basilone was one of the first well-known and celebrated American heroes of World War II. He was awarded the Medal of Honor for his heroism during the battle for Guadalcanal, in October 1942, and then ordered back home for morale-boosting parades and war bond tours. However, Basilone continually pushed to be reassigned back to combat. Eventually the Marine Corps granted his request. He was ultimately killed during the Battle of Iwo Jima.

The commander who ordered, "Take her down!"

In some of the darkest days of the Pacific campaign during World War II, an American submarine called the U.S.S. Growler surfaced after an attack, and was surprised and rammed by a Japanese warship. The Growler's captain, Howard W. Gilmore, was on the submarine's deck and realized that the Japanese ship was about to finish his crew off. Knowing he could not make it back below decks in time, he yelled an order to his second-in-command to close the hatch and dive the submarine without him, despite the fact that doing so would lead to his certain death. Gilmore perished, and he too was posthumously awarded the Medal of Honor.