Shovel (has a free version) and Diggle are the only two I know of. I don't think Digg has opened their APIs yet, so you can't digg/bury/comment from within the apps. I think the APIs are supposed to be opened soon, but not 100% sure.
There have been rumors that an official Digg app would be coming out... I've kind of been waiting on that. You would have thought Kevin Rose would have had it out a long time ago.