@mixin wpSetColWidth($col-n: 2, $gutter-width: 10px, $area-width: 760px) {
	$computed-width: floor(($area-width - (($col-n + 1) * $gutter-width)) / $col-n);
	$remainder: $area-width - ($computed-width * $col-n) - ($gutter-width * ($col-n + 1));

	width: $computed-width;
	margin: $gutter-width 0;
	margin-left: $gutter-width;
	
	&:last-child {
		width: $computed-width + $remainder;
	}
}

// So if I wanted to use this I would set up some elements I want to align in columns.

ul.columns {
	list-style: none;
	padding: 0;
	margin: 0;

	li {
		float: left;
		display: block;
		padding: 0;
		margin: 0;
		@include wpSetColWidth(3, 10px, 500px);
	}
}

// Which compiles to:

ul.columns {
  list-style: none;
  padding: 0;
  margin: 0; }
  ul.columns li {
    float: left;
    display: block;
    padding: 0;
    margin: 0;
    width: 153px;
    margin: 10px 0;
    margin-left: 10px; }
    ul.columns li:last-child {
      width: 154px; }

// See how it compensates for the uneven division and adjusts the last item in the list? Sweet.

I created this for a project I'm working on, and specifically left it bare-bones to maximise flexibility. Basically this mixin accurately calculates proper widths for each column when passed the available area, number of columns desired, and gutter width. 

Uses last-child, so it might be a pixel short in IE7 - however, the width will never be too large as it rounds down. So no float drops. Plus, who cares about IE7.. or IE at all for that matter?

Mixin for Floated Columns

SASS