From http://groundwork.sidereel.com/mixins Drop Shadow

drop-shadow

@mixin drop-shadow($color:rgba(0,0,0,0.2), $blur:0, $x:0, $y:0) { // color, blur, x, y
  -ms-box-shadow:$x $y $blur $color; // IE10+
  @if ($x == 0 and $y == 0) { // non directional glow
    filter: Glow(color=#{ie-hex-str($color)},strength=#{numericvalue($blur)});
    -ms-filter: glow(color=#{ie-hex-str($color)},strength=#{numericvalue($blur)});
  } @else {
    -ms-filter: "progid:DXImageTransform.Microsoft.Shadow(Strength=#{numericvalue($blur)}, Direction=#{degrees($x, $y)}, Color='#{ie-hex-str($color)}')"; // IE8
    filter: progid:DXImageTransform.Microsoft.Shadow(Strength=#{numericvalue($blur)}, Direction=#{degrees($x, $y)}, Color='#{ie-hex-str($color)}'); // IE7-
  }
  -o-box-shadow:$x $y $blur $color; // Opera
  -moz-box-shadow:$x $y $blur $color; // Firefox
  -webkit-box-shadow:$x $y $blur $color; // Webkit (Safari, Chrome, other)
  box-shadow:$x $y $blur $color;
}