Have extended Darren Woods (@darren) spacing mixin to include an 'all' placeholder selector.

_spacing.scss

// This is based on Nicole Sullivan's OOCSS spacing module 
// with a slight modification, namely:
//
// 1. there's no vertical or horizontal
//
// Produces the following placeholder selectors
//  %mtn {
//      margin-top: 0;
//  }
//  %mbn {
//      margin-bottom: 0;
//  }
//  %man {
//      margin: 0;
//  }
//  etc
//
// To use simple include it in your main style.scss file 
// and use the @extend directive to include in your styles
//
// Thanks to @_tim for the help with this!

$spacing_types: (m margin) (p padding);
$position_values: (t top) (b bottom) (l left) (r right) a;
$spacing_values: (n 0) (s 10px) (m 20px) (l 40px);

@each $spacing_type in $spacing_types {
  @each $position_value in $position_values {
    @each $spacing_value in $spacing_values {
      @if $position_value == a {
        %#{nth($spacing_type, 1) + nth($position_value, 1) + nth($spacing_value, 1)} {
          #{nth($spacing_type, 2)}: #{nth($spacing_value,2)};
        }
      } @else {
        %#{nth($spacing_type, 1) + nth($position_value, 1) + nth($spacing_value, 1)} {
          #{nth($spacing_type, 2)}-#{nth($position_value, 2)}: #{nth($spacing_value,2)};
        }
      }
    }
  }
}