If you have a sand bed, convict blenny's are an option as well. People always comment on mine when they see my tank. They come out when I come around as they think I am going to feed them. You can get them singly, in pairs or groups. They are cheap, mild mannered, and very disease resistant. Very much like an eel, but not escape artists and don't eat fish/inverts. Here is a picture of a group of them.
http://www.flickr.com/photos/cas_docents/4533122789/