Now I've heard of the mackerel sky and am pretty sure I know how to identify such clouds. But in looking up sleet on a weather glossary site, I came across a cloud formation of which I'd never read a description. If you didn't know this one, you might enjoy adding it to your list:

MAMMATOCUMULUS

An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud.

I'll come back in a sec and post the url, which I forgot to earmark...