Metal door frames

From a resident:

Our door frames are metal so a good supply of magnets is useful.

I have modified a dollar store shower curtain to cover my front door frame and I put it up when there are smells bothering me in the hallway. It is held in place by magnets and drags on the floor so a towel across the bottom stops the rush of air getting in there. 

I use magnets to hold up Christmas cards and other decorations around a doorway at Christmas.

You could use a magnet to hold a note on the outside of your door if you need to leave a note for someone.