What is this shell-type thing?

[Anyanka]

This is probably something really common, but my knowledge of molluscs is very limited (it is a mollusc, isn't it?). Found these on Totland Bay, Isle of Wight, earlier this week. They were attached to the underside of a piece of decayed foam, on top of a bed of seaweed.

[Jim Ford]

Goose barnacles?

When nobody knew where barnacle geese bred, people thought that they grew in the sea from goose barnacles, hence the names.

Jim

[RobSutton]

Crustaceans rather than molluscs, they float around on driftwood, or in your case the foam, filtering bits and pieces out of the sea.

[Dogghound]

The latin name is Lepas anatifera, they are common but are much commoner deeper down in the water.

[Anyanka]

Thanks for all that info - I particularly like the idea that they are the origin of barnacle geese. They do resemble a goose's head and neck a bit, and of course the colours match.