Cubes

Cubes are simplistic testing elements manufactured in the millions by GLaDOs for her test chambers.

They are designed to push down buttons.

The sentience of regular cubes is debatable, however it is confirmed that the more interesting cube, the Companion Cube is in fact sentient.

Portal 1/2
Companion Cube

Wrong Floor Cubes
All sentient cubes from the Wrong Floor Series by Zachariah Scott.

A Companion Cube appears in the episode "Wrong Floor: Love At First Scan" as a hiding place for Chell.

Video: Wrong Floor: Love At First Scan