I’ve seen everything from matchbooks to packets of sugar used to stop a table or a chair from wobbling. That’s a quick and easy fix, but to make it more permanent, you need to attach a small piece of wood, metal, or plastic to the bottom of the furniture leg.
Most home centers carry little discs to attach to the leg, but if you can’t find any, you can use wooden or plastic shims, which are pre-cut, wedge-shaped strips. All you have to do is slide the shim under the short leg until the wobble is gone. Then trace the outline of the leg onto the top of the shim using a pencil or marker.
If you cut out that profile from the shim before you attach it to the bottom of the leg, it won’t be visible when the furniture is back upright.