The Spirit of 76
This character has one of the strangest backstories out there. He was an independent comic back in the 40's but Marvel brought him to their regular lineup in Invaders #14 as part of the Crusaders.
What made his past so strange was What If? #4, "What If The Invaders Had Stayed Together After WW2?" In it, he takes on the role of Captain America after Cap disappears (or as we know it, was frozen). During a mission against a robot called Adam II, he is crushed and killed. He is later replaced by the Patriot as the new Cap.
What made this unique is that this is the only "What If?" story to ever be eventually brought into the regular Marvel universe and considered canon. The recent miniseries Captain Ameria: Patriot retold this story and gave us an idea of what happened in the years after Spirit's death.
Since we've mentioned him in the last paragraph, let's bring him into center stage. Jeff Mace was a hero who eventually found his way into the All-Winners Squad, and later leader of the Liberty Legion.
Unlike Captain America (whom he eventually replaced), Jeff had no real super powers to speak of. He was not a super-soldier, just a regular guy who wanted to fight for his country during WW2.
The recent miniseries Captain America: Patriot gave us the close-out of his career as Captain America. He passed away of cancer in Captain America #285.
This character's trip from anti-hero to hero to crazy man to anti-hero is a strange one indeed. Initially just a slightly more gung-ho version of Captain America, John Walker showed up just when Marvel was ready to take the shield away from Steve Rogers and give sales a boost (how lucky for him, eh?).
He went from the Super Patriot to Captain America for a while, before snapping because his parents were killed before his eyes. The government stepped in quietly and had Steve Rogers take the mantle again after a major fight. The Super Patriot then became today's U.S. Agent. Recently the events of Siege ended his career as a superhero when his left arm and leg were cut off by Scourge.
This character was originally an Archie comics character like Black Hood. He was eventually part of a group called the Mighty Crusaders, and has recently enjoyed a revived career thanks to DC Comics. They brought out all of the old AC heroes and have given many of them a second chance at greatness.
He actually pre-dates Captain America, though recent comics involving the character have actually been his son rather than the original. Super strong skin makes him a human shield, thus the name I suppose.
If nothing else, you have to love the guy's costume. I would have to say it's the most patriotic of the lot, better even than Captain America himself.
It wouldn't be a patriotic list if I didn't mention him. The original shield-slinger is still the best. Even though Steve Rogers has been replaced more times than I care to figure out, he always seems to find his way back in the red, white, and blue suit to save his country.
To show you how big of a cultural icon he is, when Marvel killed him off it got major news coverage. Stephen Colbert even stepped in on his show with a shield (and it still shows up in the background from time to time) to say he was taking the mantle. Of course, Steve's back (though not yet in the suit full time).
With the movie just around the corner, interest in the character should pick up again. He has a long and rich history that makes Steve Rogers the most interesting of the men to wear the mantle.
Now get out there and enjoy your day off! Happy Fourth of July everybody!