Flexible box model based grid ala bootstrap's grid semantics (`.container`, `.row`, `.span{n}`). Can set number of cols to be something other than 12.

flexgrid.scss

@import "compass/utilities/general/clearfix";
@import "compass/css3/box";
@import "compass/css3/box-sizing";

$num-cols: 12;

.container {
  @include pie-clearfix;
	margin-right: auto;
	margin-left: auto;
}

.row {
	@include display-box;
	margin-right: -15px;
	margin-left: -15px;
}

*[class*='span'] {
	@include box-sizing(border-box);
}

%margins {
	padding-right: 15px;
	padding-left: 15px;
	min-height: 1px;
}

$i: 1;
@while $i <= $num-cols {
	$percent: $i / $num-cols;

	.span#{$i} {
		@extend %margins;
		width: $percent;
	}

	.offset#{$i} {
		margin-left: $percent;
	}

	$i: $i + 1;
}