The LEGO Group

Legos are a toy by the Lego Group, seen in Toy Story 2 and Toy Story 3. There were plans for Legos to be included in Toy Story in the making of Woody's podium, but they were never seen, maybe due to copyright reasons.

Toy Story 2
In Toy Story 2, Andy has a Lego house. After Woody is stolen, Andy's toys use this in the recreation of the crime scene, only to have it accidentally destroyed by Rex.

Toy Story 3
In a the teaser trailer for Toy Story 3, the Aliens' 'Y' in Andy's Toy's Toy Story 3 logo is made out of Legos.

In the actual film, Sunnyside Daycare is full of Legos. Various vehicles and creations lay scattered around the daycare. For the first time, Lego minifigures are seen.

Later on in the film, the Time-Out Chair that Lotso's gang interrogates Buzz Lightyear in is made out of Duplo, another kind of Lego.

Trivia

 * In a twist of fate, Lego now produces Toy Story and Cars-themed sets in real life.
 * Andy's Lego house from Toy Story 2 appears on a shelf at Sunnyside, inferring that it may have been donated.