Leftover buttermilk is pretty usable, too, not to mention the remnants of a bottle of wine (but let's be honest, it's not often you'd find that in my house), or the ends of some crusty French bread.
But on a totally unrelated subject, I just read your interview on Parents Who Cook at C&Z and you mentioned that one of your kid friendly dinner staples is baked fish with buttermilk and breadcrumbs.