On the clientside, using the setshape2 value of 5 lets you define NPCs with a shape that can be interacted with but doesn't block the player.
PHP Code:
setshape2(4,4,{
5,5,5,5,
5,5,5,5,
5,5,5,5,
5,5,5,5,
});
Not insanely useful, but I found it useful when you want NPCs to interact with touching/hitting without necessarily blocking the player(like baddies, doors and such).