This package provides two functions:

• weak-ref : Creates a weak reference to an object
• weak-ref-deref : Access the object behind a weak reference

See it in action:

(setq ref (weak-ref (list 1 2 3)))
(weak-ref-deref ref) ; => (1 2 3)
(garbage-collect)
(weak-ref-deref ref) ; => nil

It's build on top of the weak references provided by hash tables.