1. Ernest Everhard (The Iron Heel, Jack London)
With a name like that, he's either a hero from a more innocent time, or a star in a certain branch of the film industry. Everhard is leading a revolution against a proto-fascist state, the result of corporate takeover, but he's not just an angry man with a gun. He's an angry intellectual with a gun, and The Iron Heel is a fantastic Marxist critique of society.
2. Uhtred of Bebbanberg, (the Saxon Stories, Bernard Cornwell.
Uhtred of Bebbanberg is a man torn between two worlds -- Anglo-Saxon by birth and Viking by sympathies. Kidnapped from his family's estate by the Vikings who razed it, Uhtred delights in the Norse's unapologetic rivalry and despises the pious misery of the Anglo-Saxons. Service to the English king (Alfred the Great) is his only path to reclaming those family lands, however, and so he exists as a man truly loyal to no one but himself. Given the treachery to be found on either side, that's probably the best thing to do.
3. Alexander Til (The Revolutionist, Robert Littel)
Xander Til was just a boy when his parents and he emigrated from Russia, but now as a passionate young man he's on his way home. America is not the promised land for Til and his neighbors, and back at home the people are rising in fury against the Tsarist government. Til becomes a leading Bolshevik, but quickly realizes the drivers of this revolution are just as corrupt as the men they fight against.
4. Jefferson Davis Bussey (Rifles for Watie, Harold Keith)
Jefferson Davis Bussey is, contrary to his name, a Union man. His family is sternly anti-slavery, and he lies to the recruiting office in order to don the Union blue and fight against the wretched men who want to bring slavery to Kansas. When a scouting mission goes awry and Bussey is forced to pose as a Confederate soldier to save his life, he learns that the men who fight for the legendary general Stand Watie are fighting not to expand slavery, but to establish their own nation -- for Watie is a Cherokee.
5. Sirius Black (The Prisoner of Azkaban, Goblet of Fire, Order of the Phoenix; J.K. Rowling)
Raised to be a hateful aristocrat, Black rejected his family in favor of hanging out with his half-blood friends, one of whom was a werewolf. He remains one of the series' favorite characters as a foster parent to young Harry.
6.Charles Croker (A Man in Full, Tom Wolfe)
Charlie Croker turned his back on a life of wealth and influence to become a Stoic evangelist, which is odd enough that I think I'll just leave it there.
7. Michael Brock, The Street Lawyer. John Grisham
Michael Brock is your standard overworked, overpaid, unhappy lawyer until a homeless man takes him hostage. After the man is shot by a police sniper and leaves portions of his brain on Brock's new coat, he's bothered to the point that he begins serving the needs of the poor and homeless as a lawyer working for a nonprofit. In short time he loses his wife, but gains a lot more.
8. Huckleberry Finn (The Adventures of, Mark Twain)
"All right, then. I'll go to Hell," Finn says after being forced to choose between doing the human thing (being loyal to his friend) and the social/culturally-accepted thing of turning his friend Jim in as an escaped slave.
9. Richard Sharpe (Sharpe's Series, Bernard Cornwell)
I almost feel like I'm cheating because I've used one of Cornwell's figures before. but Sharpe is a lovable loose-cannon character who remains a soldier because he's good at it -- not because he thinks King George deserves his service. Indeed, he's liked many of his enemies more than his bosses.
10. Scout Finch, (To Kill a Mockingbird, Harper Lee)
Looking back on this list I realize I read far too few books with female heroes, but I'm happy to include Scout. Despite being raised in a culture that encourages subordination and meekness among its women, Scout is marvelously pugnacious and self-willed. She's a real credit to her father.