Originally Posted by hompac
Simply remove one of the hinge pins. Lay it on a hard surface (sidewalk, basement floor) and hit it with your hammer causing a slight bend in the pin. Replace the pin and the door will stay where you put it.
Or if you have a dead pet just laying around the house you can use that as a door stop.
But seriously, yes, your door is not plumb. Sometimes when an inexperienced door installer installs a door, they are more concerned with having an even 'reveal' (even gap) around the door, than having that AND having it plumb and level. Gravity will cause an unplumb door to try and shut(or open) on its own. The Best solution is to reinstall it correctly. If you try to adjust the bottom hinge out to rectify this, then you will probably have the door slab hit the jamb on the opposite side. If the installer only 'pin nailed' or tacked the door in, you may be able to do this, and then use a block of wood to offset the opposite side, but you will also have to use that same block against the corner diaganally opposite from that same hinge.
Not sure if I described this well, but if not just ask.