Every interaction between Christopher and his son, specifically when Wikus is hiding in Christopher's shack and asks him why his son is staring at him. Christopher's answer? "He likes you."
When Christopher's son compares his arm with Wikus'.
The last shot in the movie is a fully transformed Wikus making a flower for his wife out of a tin can. Awwwww.
Wikus's wife holding out hope for his return (keeping his office intact, holding on to every memento from their life together, saving the flower he made for her) despite her friends' insistence that he must be dead.
Wikus's second phone call with his wife. After the first phone call about the MNU's lies about Wikus's alien sex, the second phone call reveals that Wikus's wife realises they're lies, and she tearfully asks Wikus when things can return to normal again.
Wikus's way to snap Christopher out of his Heroic BSOD: "Think of your son."
"We go home now?" "Yes."
"Three years. I promise."
Wikus runs away in the mech; Christopher has a gun to his head; Wikus comes back.
A dark example, but the other prawns tearing Koobus into pieces to save Wikus kinda makes you smile, for more than one reason.
Wikus showing a documentary-maker a picture of his wife on their wedding day, and referring to her as his "special angel."
The look on Wikus' face during the final scene as he watches Christopher and his son taking off in the mothership. He knows he's probably doomed, and he knows he's redeemed himself for his cowardice. A big old Tear Jerker as well. One of the most powerful shots in the film, and there are many.
One of Wikus' co-workers released official records regarding everything the MNU was doing at District 9. He states that when he went after the records, they weren't locked away, or decrypted, or anything, just sitting there for anyone to take... probably because everyone knew what would happen to them if they publicly released them. His co-worker, knowing that if he released these records, would be lucky if all they did was put him in jail... released them anyways without hesitation.