This is interesting. I'll start with one. I learnt this last week whilst in Shakespeare's house in Stratford Upon Avon.
Apparently we say 'sleep tight', because back in t'day the pull out bed from under the main bed supported the person by a series of crossed over ropes. These ropes had to be re-tightened by turning a large key attached to the end of it every evening before bed as it would sag throughout the night. Therefore, saying sleep tight is wishing someone a comfortable night that is undisturbed by the loosening of the ropes.