A variety of snail species (ie. Margarita, Trochus, Turbos, Ceriths, etc.), conchs (depending on the size of the tank), hermits, cukes, etc. Pods and bristleworms are helpful too, but are generally already present in your system if you've introduced some good quality live rock and/or live sand.
The general consensus seems to be to avoid sand sifting stars and fish as they will prey upon the useful sandbed critters.
HTH
