The rear wheel on my bike recently became loose and wobbly. I removed the freewheel and found that the ball bearing cup completely broke away from the hub, so the axle was entirely unsupported on one side.
Instead of going through the trouble of replacing the hub or getting a new wheel, I made a quick fix by stacking a few washers over the hub, putting the freewheel back on to hold them in place, and then packing the balls inside along with plenty of grease. I added another washer between the cone and lock nut to keep out water and dirt. You also have to adjust the spacing of the lock nuts from both ends of the axle so that the wheel remains centered.
The wheel spins surprisingly smoothly, but of course this hack will most likely not last very long since the washers and interior of the freewheel are not designed to function as ball bearings.
The diagram is cropped from this Wikipedia image: http://en.wikipedia.org/wiki/File:Labeled_Bicycle_...