Hexagons
... and other polygons
Hexagons and other polygons, like octagons, are often used to form a boundary for logos.
These figures are always apportionable into triangles, squares or rectangles. The breakdown of these shapes often create interesting logo creations, which can't just be used as a boundary anymore. Most times, though, these shapes are used as a limitation, with a character, figure or the like in the inside.
These polygons always have a base, so they appear balanced and relatively stable.