Any of the gobies of the Valenciennea family (Sleeper Gobies)are excellent at keeping the substrate clean.
Valenciennea puellaris - Orange spotted Sleeper Goby
V. strigata - Yellow headed sleeper goby
V. longipinnis - Long finned Sleeper Goby
I got my orange spotted sleeper goby from Salwaterfish.com .
Be warned though -they sift through the sand looking for critters to eat. So if you have a live sand bed with pods and such - they will eat them. If you have crushed coral, it may be too coarse for them - injuring their mouths and gills.
Other than that, hermit crabs are excellent for cleaning the substrate.
Also, if you get a sleeper goby, you must keep the tank covered. They will jump out. I have lost a couple this way.
[ August 08, 2001: Message edited by: Amphiprion ]