In every Pixar film, you'll find at least one reference to another Pixar movie, but usually there are a whole lot more. I've gathered some of my favorites, but they're countless. If I've missed your favorite, share it in the comments!
1. Pizza Planet. Pizza Planet turns up in just about every movie, if you look closely enough. In The Incredibles, a delivery truck can be spotted in a scene where the family is driving on the highway, and EVE from WALL-E checks the engine of a Pizza Planet vehicle to see if it contains any signs of plant life.
2. In Finding Nemo, play close attention to the scene in the dentist's office. A Buzz Lightyear toy can be seen in the waiting room.
3. Also, in the same scene, the mobile that you see in the dentist's office is also featured in Boo's room in Monsters, Inc., as are a Nemo toy and a Jessie toy (from Toy Story 2). And, check out the comic book the little boy is reading "“ it's The Incredibles.
4. Ratatouille features a guest appearance from Incredibles nemesis Bomb Voyage "“ he has a brief cameo as a mime.
5. All of the books on Andy's shelf in Toy Story have titles that are actually the titles of Pixar shorts.
6. Dinoco, like Pizza Planet, is another fictional company in the Pixarverse that appears in multiple movies. The oil company is first seen in Toy Story as a little gas station, then in Cars as a sponsor of the Piston cup. It's even in WALL-E - the name and logo can be seen on a lighter.
7. References to A113 shouldn't be too hard to find "“ they're scattered all over every Pixar movie. A113 is the room at the California Institute of the Arts where character animators trained. This isn't exclusive to Pixar "“ you can find A113 references all over "“ even on The Simpsons. A few of the Pixar cameos include Mater's license plate in Cars, the license plate number on Andy's mom's van in Toy Story and Toy Story 2, as the number on the tag of a lab rat in Ratatouille and as a code in WALL-E.
8. In Cars, the Piston cup cars all have "Lightyear" instead of "Goodyear" written on the tires. Sneaky.
9. WALL-E works in a clever reference to Ratatouille - the mechanic mice are called REM-E.
Check out more interesting references (and pictures) here.