This is something that has bugged me for awhile, and it came to a head during a discussion in the comment section of my Watchmen review over at Huffington Post. Basically, the debate was whether or not The Incredibles had stolen much of its storyline from Watchmen. The answer is most likely 'probably', but you could make that argument for most Pixar cartoons over the last fourteen years. Whether this lessens the esteem of Pixar is a question in and of itself, but it's worth noting just how unoriginal many of their most popular animated films really are.
Toy Story (1995) - The 'one that started it all' was a wonderfully entertaining and splendidly acted fable about the secret world of toys, toys that came to life when the children were not around. However, there is trouble in Andy's room. Every birthday, all the toys lie in fear of being replaced by a newer, shinier toy, and thus being regulated to storage or worse. Tragedy strikes when Woody, the favorite wind-up cowboy doll, fears replacement by the new and fancy space hero Buzz Lightyear. In a crazed panic, Woody causes Buzz to fall out of the window, thus placing him in mortal peril. Can Woody overcome his jealousy and save Buzz?
Sound familiar? Well, if you've seen Jim Henson's The Christmas Toy, it just might. The 1986 puppet story debuted on television as a forty-five minute Christmas special. In the world of The Christmas Toy, all of the toys come to life when people are not around. And Christmas is a time of fear and trepidation as every toy awaits Christmas morning when they discover if they will be replaced by a newer, shinier variation of themselves. This year, Rugby, the favorite tiger, is deathly afraid of being replaced by Meteora, a She-Ra type action doll. Panic ensues when Rugby lets Meteora out of her box prematurely, and the gang must team up to get Meteora back in her proper place before Christmas morning.
Do the similarities matter? Yes and no... while this is one of the most obvious examples of 'accidental plagiarism', Toy Story is still a better film in every way. The animation is still impressive, the acting is peerless (I'd argue any day of the week that the role of Woody represents Tom Hanks' finest work), and the film sets the Pixar template for gently humorous comedy and open-hearted emotionalism. That it is so similar to The Christmas Toy is disconcerting, but if anything the similarities will cause more people to search out the little-remembered Jim Henson original.
A Bug's Life (1998) - The second of two animated insect movies to debut in the latter months of 1998, this film isn't so much an apparent rip-off of a previous work so much as a reworking of a well-worn thematic template. This story of an outcast bug who unknowingly recruits a group of circus performers to defeat a monstrous grasshopper who menaces their village, is simply a variation on The Seven Samurai, The Magnificent Seven, and Three Amigos (to say nothing of the masterpiece variation on this template, Galaxy Quest, which followed a full year later).
Do the similarities matter? Not in the least. While A Bug's Life may be the most conventional of all Pixar pictures, and one of their middle-of-the road efforts, it does boast gorgeous animation and fine vocal acting by Dave Foley and Kevin Spacey. It also climaxes with a stunningly intense action chase scene that came along when examples of action set pieces in American animated features were all too rare. And again, this is an example of using a timeworn myth, rather than seemingly copying a specific prior film.
Toy Story 2 (1999) - Still the crown jewel in the Pixar cannon, this is the rare complete original in their filmography. I seriously cannot find any films that this takes any major chunks of its narrative from. It may be a coincidence, but the very best Pixar cartoon is also their most overtly original.
For the rest of the Pixar cannon, including one entry that's such a carbon copy of another film that they could have gotten sued, read Mendelson's Memos.