We use Brownian dynamics simulations of colloidal systems to explore new nonequilibrium collective e?ects. Colloids driven over a symmetric ?ashing periodic substrate undergo a ratcheting behavior which can be used to fractionate di?erent particle species. The addition of nondriven colloids can increase the drift velocity of the driven colloids due to collective interactions. We show that colloids in optical trap lattices can be used to create an arti?cial spin ice system which shows a rich variety of ice rule and other orderings. Our study of an interstitial point defect in a two-dimensional colloidal crystal shows that it is more mobile than a vacancy defect due to the more two-dimensional character of the di?usion. We also demonstrate that a bidisperse, phase segregated colloid system can be mixed e?ciently in the absence of temperature by driving the colloids over a random substrate and inducing plastic ?ow.