What also works is the shelf "paper" they sell at the dollar star that's rubberized feeling. It comes in a roll. I cut a piece off the roll and put it under all my machines - it keeps them from wobbling around. I also put a piece under my foot pedal holder to keep it from sliding around. If you have a machine with a clutch that has to be turned to disengage it in order to wind a bobbin, a piece gives you a solid grip when you're trying to disengage (a rubber band around the clutch wheel works good too).
I also use it under my cutting boards to keep them from sliding around, and anything else that I need to stabilize.
It's so cheap that when it gets really dirty or discolored, just toss it and cut a new piece.