From DIYinfo.org
When the bottom corner of the door rubs on the floor, the cause is either faulty hinges or loose joints in the door.
Partly open the door and lift the handle or knob to see if there is movement at the hinges or joints.
Faulty Hinges
If the hinges are loose, remove them and plug the holes with dowels. Refit the screws, or fit thicker screws. If necessary enlarge the countersunk holes in the hinge to take the larger screw heads. Use a high-speed steel bit.
If the movement is in the knuckle of the hinge due to a worn hinge pin the only cure is to fit new hinges. It may be that the hinges are not big enough to support the weight of the door. In this case fit larger, heavier hinges and perhaps a third hinge midway between them.
Loose Door Joints
If the door joints are loose, glue and cramp them back into place.
1 Take the door off its hinges, and try to dismantle it by gently prising the loose joints apart.
2 Re-glue the joints and put them together again. If they will not come apart, inject woodworking glue into the joints
3 On the edge of the door, drive small wooden wedges into the ends of the tenons to prevent the joints from opening up again
4 Drill through the face of the door and through the tenon and drive a dowel smeared with woodworking glue into the hole. This will lock the tenon in place. On an exterior door use waterproof glue.
5 Cramp the door with a sash or web cramp while the glue sets. See Clamps Types And Uses
6 Trim off the dowels flush with the door.