The ornamental design for a free-standing sign, as shown and described.
FIG. 1 is a front, perspective view of a free-standing sign showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof; and,
FIG. 6 is a top view thereof.
Any bottom surfaces of the free-standing sign are not claimed, and therefore, the bottom view has been intentionally omitted. The features shown in broken lines depict unclaimed, optional portions of the free-standing sign only and form no part of the claimed design.