Riding in this morning on the freshly deskogged MTB I considered the chainstay protector I had applied. One length of dead inner tube, wrap the stay and cable tie it at either end. Cheap, practical, and as effective as the expensive alternatives. Doesn't even look particularly bad.
I then considered the flower stick (built from inner tubes), the endless supply of rubber bands (cut from inner tubes), the Heath Robinson rig holding the cat flap open before the mogs worked out how to open it themselves (made from inner tubes) the flogger (made from inner tubes), the frame padding for when the bikes go on the car roof (cut from inner tubes)...
Is there any more useful defunct bicycle bit?