An animated SVG, suitable for use as a progress indicator.

spinner.svg

<svg 
  version="1.1" 
  xmlns="http://www.w3.org/2000/svg" 
  xmlns:xlink="http://www.w3.org/1999/xlink" 
  height="320"
  width="320"
  viewBox="0 0 16 16" 
  xml:space="preserve" >
  <path fill="#000" 
     d="M  8,  0
        A .5, .5,  0,  0,  0,  8,  1
        A  6,  7,  0,  0,  1, 14,  8
        A  6,  6,  0,  0,  1,  8, 14
        A  5,  6,  0,  0,  1,  3,  8
        A  1,  1,  0,  0,  0,  0,  8
        A  8,  8,  0,  0,  0,  8, 16
        A  8,  8,  0,  0,  0, 16,  8
        A  8,  8,  0,  0,  0,  8,  0
        Z" 
    >
    <animateTransform
      attributeType="XML"
      attributeName="transform"
      type="rotate"
      from="0 8 8"
      to="360 8 8"
      dur="0.6s"
      repeatCount="indefinite" />
  </path>
</svg>

Two easy ways to change the size: alter height and width in the svg tag, or scale it as a background image in the CSS selector of your choice.

Kent Brewster you're my hero.

Minor but important update for Firefox: it wants that leading zero in dur="0.6s"