The famous Japanese fairy tale “Peach Boy” (“Momotaro” in Japanese) has an elderly childless couple adopting the eponymous hero as an infant. As far as I can tell, they were good parents to him.
ETA: Ooh, someone made a list, though it doesn’t seem to have Peach Boy on it.
Thumbelina?
Good adoptive parents from non-fairytales:
Marilla and Matthew Cuthbert (Anne of Green Gables)
James Alden (Boxcar Children)
Alec Campbell (Eight Cousins)
Abigail Tillerman (Homecoming)
Oh, man, the Dope is awesome!
Arthur/Ector and …maybe Thumbelina (on googling, not entirely sure she had good adoptive parents exactly?) are exactly the kind of thing I’m looking for, although I’m happy to get non-fairy-tale/mythic literature as well. Thank you! More? 
Again as a non-fairy-tale story: Noel Streatfeild wrote several books for children that feature good adoptive or foster parents. The one I’m familiar with, Ballet Shoes, has three orphaned girls who are almost “collected” by an eccentric fossil collector, who then disappears on an expedition for years. The girls are raised by the great-niece he himself had been a kind if inept foster parent to, and her nurse, who stays on even when the money runs out and she isn’t paid any more, because she cares so deeply for the girls. They are also assisted by a number of other kindly adults who come to live as boarders in the family’s enormous house.
(Well, one of the girls isn’t technically an orphan. She is given up by her impoverished mother, a dancer. She comes to the house with a note that concludes: “Her name is Posy. Unfortunate, but true.”)
And the supervillain from Despicable Me, although it takes him awhile to come around.
Joseph.
If we turn to Ancient Greece I can think of an example or two.
Oedipus has decent adoptive parents. Too bad things worked out the way they did with his birth parents.
Heracles had an evil stepmother, but seemed to get along well enough with his stepfather Amphitryon.
Captain Hook, in Hook.
Uncle Owen and Aunt Beru did a good job raising the kid – not a blood relative, even – who helped that crazy old wizard rescue the beautiful princess from the black knight.
(um… spoilers.)
I would be willing to consider this, but it’s highly debatable on the face of it. If he were actually a decent step-parent/foster parent, he would have tried harder with the girl, instead of leaving her locked in a tower like Rapunzel. Granted she wasn’t being very cooperative, but that’s on the adult to mend, not the kid.
He was nice to Jack, but that wasn’t really for Jack’s benefit. I would be willing to grant that he became fond of Jack over time, but that could also be an outreach of his narcissism, seeing Jack as a little version of himself, which would be the only thing Hook *could *actually love.
But casting him as a “good” foster parent is a stretch - especially since the kids were *kidnapped *for the express purpose of forcing their father to come to fairyland and do battle for them, not because Hook had some octogenarian urge to have kids all of a sudden.
Awesome. Joseph, ha! And Oedipus and Luke… that’s again exactly the kind of thing I was looking for. And I had never heard of the Gullfaxi/Gunnfoder one, that’s great! I do appreciate the non-myth-fairy ones too, though… I need to go pick up my copy of Ballet Shoes again, and reread the Tillerman saga…
Roald Dahl’s Matilda? Maybe not formally adopted, but her ‘happy ending’ is being fostered by someone much nicer than her birth parents.
Pick, pick, pick… he listened to Jack, he never lied to him (the speech about how his parents were happier and had more fun before they had kids was freaking AMAZING), he went to his ball game… sure, there was the felony kidnapping, but the way he treated him and cared for him AFTER the kidnapping is miles ahead of the way step- or adoptive-parents are portrayed in your typical fairy tale.
Not a fairy tale, but in the Disney version of Hercules, Amphitryo and his wife were wonderful adoptive parents… not a cuckold and a wife who got pregnant while playing around with Zeus.
Chiron the centaur.
Pharoah’s daughter, who found Moses in the bulrushes.
The Lady of the Lake, who raised Lancelot.
The abbess who raised Galahad.
Tom Sawyer’s aunt.
The dwarf Regin who brought up Sigurd
at least until
[spoiler]
He developed gold fever and tried to kill Sigurd for Fáfnir’s hoard [/spoiler]
In the original versions he is still alive.
Also from the sagas:
Thord from Laxdaela saga, the foster father of Olaf Hoskuldsson (Olaf The Peacock)
Heimir, Brynhild’s foster father (and later also her daughter Aslaug’s foster father) in Volsunga saga and Ragnars saga loðbrokar.
Regin (again) Helgi and Hróar’s foster father in Hrolfs saga kraka.
Snorri Goði in Laxdaela saga.