1. wiki : caster
2. lego
– HiTechnic Rotacaster
– Caster wheel for NXT
– Lego PC Bot Castor
– 10 Cool Lego Mindstorms: Ultimate Builders Projects => Caster wheel
-> do not use coupled wheel, use only a single, freely rotating wheel
– The Art of LEGO MINDSTORMS NXT-G Programming : caster wheel for nxt
– Building Robots with LEGO Mindstorms NXT : Caster wheel
– Omnidirectional wheel
– Tri-caster
– Caster wheels and gears