If you live in an older home you will sometime have the problem of doors slowly swinging shut. This can be caused by shifting foundations or out-of-square door opening.
This tip comes from the March 2011 edition of This Old House Magazine.
A door swings open or closed on its own. Pull out one of the hinge pins, lay it on a sturdy work surface, and hit the midpoint of the shaft with a hammer. Then reinsert the pin; the blow will have bent it slightly, providing enough resistance to prevent the unwanted movement.