Last time I stayed at a hotel, the do not disturb sign was taken as more a suggestion than a requirement by the hotel staff. The DoorKnob condition, really weird name, takes the knob and actually retracts it into the door so the knob can’t be turned.
From the picture here there appears to be a slot under the knob for a hotel key card. This is a mere concept from Yanko Design, but it looks pretty slick.
It could be useful in the dorm too if you are “busy” and don’t want someone walking in on you. I guess you could put t timer out there too so they know when to come back, but it might bruise your ego if you don’t use all your time. Via OhGizmo!