A Swedish company called Ixbari Solutions has come up with an invention called WC-Avoider that allows people to discreetly notify others to avoid a "recently used" restroom. We've all seen the indicators on restroom doors that indicate "Vacant" or "Occupied." The WC-Avoider offers a third setting, "Recently Used."
Upon exiting a restroom, a person would have the option of pressing a button or turning a handle inside the restroom door to activate the "Recently Used" setting, thus avoiding the situation of either making an awkward announcement or simply ignoring the issue.
While this invention could be used in public restrooms in all sorts of settings, I think it could be especially useful in the workplace. People spend more and more time working at all hours, and this invention helps restore a little more of a sense of privacy among coworkers.