Two patterns from Bill Venners; I think both are heavily used in ScalaTest:

Stackable Trait (similar in structure to the decorator pattern, except it involves decoration for the purpose of class composition instead of object composition).

Selfless Trait (allows library designers to provide services that their clients can access either through mixins or imports).

Type safe builder

Independently Extensible Solutions to the Expression Problem - just like the "Scalable Component Abstraction", it's not a pattern catalog, but it also deals with similar problems (e.g. the Visitor pattern)

Deprecating the Observer Pattern - an alternative to the Observer.

We can also consider the Scala emulation of Haskell type classes a design pattern. The first description (that I could find at least) is in Poor Man's Type Classes. Quite some blog entries are also available with this topic.

And I think I'm not completely wrong if I also mention the various monads. You can find a lot of resources dealing with them.