This note describes how to use an omni-directional wheel as a caster. The omni-directional wheel is unique because it rolls freely in two directions. In one direction, it rolls like a normal wheel. It can also roll laterally because of the smaller wheels spread about its circumference. Many robots use a 2-motor differential steering system with a caster.
Hardware stores sell a wide variety of casters, but most are too big for small robots.
Furthermore, the casters in hardware stores are designed to withstand large forces so they tend to have fairly coarse motion. Small robots usually exert a small amount of force on a caster. The caster must turn and roll easily to be effective. One alternative to a hardware store caster is making one from scratch. However, this takes a significant amount of work and experience.