What are the similarities between Boo Radley and Tom Robinson in Lee's To Kill a Mockingbird?

On the surface, there are not many similarities between Boo Radley and Tom Robinson.  However, if you look closely, they are both symbolically compared to mockingbirds.  Atticus tells his children it is a sin to kill a mockingbird.  Mr. Underwood compares Tom Robinson’s death to the senseless killing of songbirds, and Scout compares Boo Radley to a mockingbird.  Each of these men is a victim of society in a different way.  Tom Robinson was targeted because of his race, and Boo Radley for his eccentricity. 

The story of Tom Robinson is more serious.  As an African-American, Tom Robinson was constantly victimized.  When Bob Ewell saw him with his daughter, he cried rape.  The story of a white woman and a black man could not be tolerated.  Robinson was convicted even though he was innocent, and Atticus proved during the trial that no crime was committed.  Tom Robinson had one useless arm and could not have caused Mayella Ewell's injuries. 

The worse part is that Tom Robinson committed suicide in prison.  Since he was crippled, he was shot attempting escape.  Even Mr. Underwood, the town's racist newspaperman, considered Robinson's death a travesty.

Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom’s death to the senseless slaughter of songbirds by hunters and children ... (Ch. 25)

Tom Robinson never did anything bad to anybody.  All he did was try to help Mayella.  For his trouble, he was arrested, tried, convicted, and shot, all because of the color of his skin.

Boo Radley is the Finches' reclusive neighbor.  He never comes out of his house, and Dill and the other children decide that he is lonely and they need to make contact.  They try getting a glimpse of him or leaving him notes.  Atticus warns them to leave him alone.  He feels that Boo and his family deserves privacy.

The children make progress though.  They bring him out of his shell, and he actually starts leaving them gifts.  He puts presents in a tree hollow, mends Jem's pants and leaves them for him when he loses them, and puts a blanket on Scout's shoulders secretly during the fire.  His final gift is to rescue the children from Bob Ewell.  

Heck Tate told Atticus that he was not going to tell anyone what Boo Radley did, to save Boo from the notoriety and nosy neighbors.  When Atticus asked Scout, she agreed.

“Yes sir, I understand,” I reassured him. “Mr. Tate was right.”
Atticus disengaged himself and looked at me. “What do you mean?”
“Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?” (Ch. 30)

Boo Radley was a mockingbird because he was a victim of society's cruelty.  The children saw something in him that no one else did, and became his only friends.  He repaid their friendship by saving their lives.