These things are amazingly versatile, easy to find, and uh... free. To acquire some, simply lurk around some alleys in the middle of the night, particularly in the restaurant district. They ought to be found placed near the rear doors, grab only what you need, sneak home.

They may be stacked vertically with the open side facing away from the wall to form a cubbyhole sort of shelving book case, or simply as a little box/drawer to store things in, such as socks. Another use is implementing them as a stool/ladder of adjustable height, due to their rugged and highly stackable construction. Graffiti writers favor crates when painting trains to compensate for the high ground clearance of most boxcars, plus they are light and easy to carry. This only begins to touch on the possibilities really.

Oh, and please avoid the sour attempts of department stores to capitalize off the milk crate versatility factor by selling faux versions, they are of vastly inferior construction and often fail to stack properly.