Handy dandy SASS mixins. Some I wrote myself, others I've modified and the rest I've taken. Enjoy.

handy-mixins.scss

// MATH & LAYOUT
// --------------------------------------------------
@function calc-em($target-px, $context: $base-font-size) {
  @return ($target-px / $context) * 1em;
}

/* One-true layout hack for equal height columns */
@mixin equalize-column {
  margin-bottom: -99999px;
  padding-bottom: 99999px; 
}

// Must be using Susy grid
%container {
  @include container;
}

// Clearfix
%group {
  *zoom:1;
  &:before,
  &:after {
    visibility: hidden;
    display: block;
    font-size: 0;
    content: ".";
    clear: both;
    height: 0;
  }
}

// Absolute Positioning
@mixin abs-pos ($top: auto, $right: auto, $bottom: auto, $left: auto) {
  top: $top;
  right: $right;
  bottom: $bottom;
  left: $left;
  position: absolute;
}

// MEDIA QUERIES
// --------------------------------------------------
// Assumes you want IE 8 and below to use largest screen size
@mixin respond-to($viewport-width, $ie-version: 'lt-ie9') {
  @media only screen and (min-width: $viewport-width) {
    @content;
  }
  @if $ie-version == 'lt-ie9' { 
    html.lt-ie9 & { // Using Paul Irish's conditional comments
      @content;
    }
  }
  @else if $ie-version == 'lt-ie8' {
    html.lt-ie8 & { // Using Paul Irish's conditional comments
      @content;
    }
  }
}

// Retina Image
@mixin image-2x($image, $width, $height) {
  @media (min--moz-device-pixel-ratio: 1.3),
         (-o-min-device-pixel-ratio: 2.6/2),
         (-webkit-min-device-pixel-ratio: 1.3),
         (min-device-pixel-ratio: 1.3),
         (min-resolution: 1.3dppx) {
    /* on retina, use image that's scaled by 2 */
    background-image: url($image);
    background-size: $width $height;
  }
}

// TYPOGRAPHY
// --------------------------------------------------
// Replace logo with text
@mixin replace-text($width, $height, $background-details) {
  text-indent: -9999px;
  white-space: nowrap;
  overflow: hidden;
  display: block;
  width: $width;
  height: $height;
  background: $background-details;
}

// Creates ellipsis for overflow text
@mixin ellipsis() {
  overflow: hidden;
  white-space: nowrap;
  text-overflow: ellipsis;
}

// ANIMATIONS
// --------------------------------------------------
@mixin keyframes($name) {
  @-webkit-keyframes #{$name} {
    @content
  }
  @-moz-keyframes #{$name} {
    @content
  }
  @-ms-keyframes #{$name} {
    @content
  }
  @-o-keyframes #{$name} {
    @content
  }
  @keyframes #{$name} {
    @content
  }
}

@mixin animation($value) {
  -webkit-animation: $value;
  -moz-animation: $value;
  -ms-animation: $value;
  -o-animation: $value;
  animation: $value;
}
// Usage 
// **************************************************
/*
@include keyframes(move) {
  0%   { left: 0; }
  100% { left: 100px; }
}

.box {
  @include animation(move 0.5s ease infinite alternate);
}
*/

// IE HACKS
// --------------------------------------------------
@mixin ie7 {
  * + html & { // Using browser hacks
    @content;
  }
}

@mixin ie8 {
  html.ie8 & { // Using Paul Irish's conditional comments
    @content;
  }
}

@mixin ie9 {
  html.ie9 & { // Using Paul Irish's conditional comments
    @content;
  }
}