If you have a really old house, your floors may not be perfectly horistontal, and some of your furniture feet may need a slight extension for the furniture to sit straight. Instead of stacking coins, paper or small wood chips, why not make something that is suited for the purpose. The STLs herein are just examples. Use the Customizer to make designs that fit your needs.
This design only works for rectangular feet. The hole in the middle is optional.
Do print with (close to) 100% infill for strength.