Ratatouille: When Remmy is scurrying through the walls and enters an apartment, Dug from the movie Up starts barking at him. We only hear him barking and see his shadow on the wall. Also, during the scene when the dwarf-sized chef is chasing Remmy on a motorcycle because Remmy snatched the papers, who can eye-spy a poster of Mr. Incredible if you looked extremely closely.
Nemo: Mr. Incredible also is noticed in this movie. When the pelican is attacking the dentist and a boy is watching in shock, you can see him reading a Mr. Incredible comic book. When Nemo enters the aquarium, you'll notice that the dentist's office has a toy box with Buzz Lightyear inside. During the credits, Mike Wazouski the little green cyclops is scuba diving.
Monsters Inc.: When Sulley returns Boo to her door, Boo shows him a toy Nemo and Jessie.
Frozen: When guests are gathering for Elsa's coronation, Flynn and Rapunzel with short, dark hair are noticed. When Anna goes off in search of Elsa, she rides Maximus the horse from Tangled.
Up: When the house takes off and a little girl is watching from her bedroom window, try to eye-spy a Lotso teddybear in her bedroom.
Toy Story 3: When the toys arrive at Sunnyside and enter the Butterfly Room, perhaps Boo from Monsters Inc. is a little girl playing with some Daycare toys. Plus, Totoro (who is not a Pixar character) is one of Bonnie's toys. Sid from Toy Story 1 is the garbageman.
Everyone knows Barbie is in both Toy Story 2 and 3.
Tin Toy: Big Baby from Toy Story 3 is in that short film.
Toy Story 2: The cleaner who fixes Woody's ripped arm is the old man playing chess from the short film Gerri's Game. He also looks like Madame's lawyer, George, from Walt Disney's The Aristo Cats.
Tangled (one character in it's own movie): During the opening scene, you can see a toy chameleon on Princess Rapunzel's mobile when she is a baby.
In most of the movies and short films, the yellow ball with the blue stripe and the red star from Toy Story is found, like in the Pixar short about the big and little studio lamps who appears in the Pixar logo.