Image by 0olong
Mamma, or mammatus: relatively rare clouds which usually form when pockets of colder air drop from the base of a particularly unstable cumulonimbus. They are associated with particularly violent storms.
Another old, scanned photo of mine (hence that white line, which I suppose there’s probably a Photoshop filter somewhere to deal with?) – as I recall, this one is from when I was living in Essex, and the storm (shortly before sunset) hadn’t quite hit yet.