My collection of defaults and mixins for less and underscore in coffeescript.

mixins.coffee

_.mixin
  isEmail: (string) ->
    /^[a-z0-9_.%+\-]+@[0-9a-z.\-]+\.[a-z]{2,6}$/i.test(string)

  isUsername: (string) ->
    /^[\w\d\.-]{1,15}$/i.test(string)

  isName: (string) ->
    /^(?:[\u00c0-\u01ffa-zA-Z'-]){2,}(?:\s[\u00c0-\u01ffa-zA-Z'-]{2,})+$/i.test(string)

  toSlug: (string) ->
    string.toLowerCase().replace(/-+/g, '').replace(/\s+/g, '-').replace(/[^a-z0-9-]/g, '')

  isDate: (year, month, day) ->
    year  = +year
    month = +month
    day   = +day
    date  = new Date(year, month, day)
    return false if isNaN(date.getTime())
    return date.getFullYear() is year and date.getMonth() is month and date.getDate() is day

  isObjectID: (string) ->
    /[0-9a-f]{24}/.test(string)

  formatCurrency: (num) ->
    num     = num.toString()
    dollars = num.slice(0, -2)
    cents   = num.slice(-2)
    dollars = dollars.replace(/\B(?=(\d{3})+(?!\d))/g, ",")
    "$#{dollars}.#{cents}"

mixins.less

// Media Queries
@media_desktop: ~"only screen and (min-width: 1025px)";
@media_tablet: ~"only screen and (min-width: 768px) and (max-width: 1024px)";
@media_mobile: ~"only screen and (max-width: 767px)";
@media_retina: ~"(min-resolution: 120dpi), (-webkit-min-device-pixel-ratio: 1.25), (min--moz-device-pixel-ratio: 1.25), (-o-min-device-pixel-ratio: 4/3), (min-device-pixel-ratio: 1.25)";

// Clearfix mini
.clearfix() {
  zoom: 1;
  &:before, &:after {content: ''; display: table;}
  &:after {clear: both;}
}

.triangle(@width, @color) {
  content: '';
  position: absolute;
  border-style: solid;
  border-width: @width;
  border-color: @color;
  .size(0px);
}

.vertical-center(@height, @position: absolute) {
  position: @position;
  top: 50%;
  margin-top: -@height/2;
}
.clearfix() {
  zoom: 1;
  &:before, &:after {content: ''; display: table;}
  &:after {clear: both;}
}

.circle(@size) {
  border-radius: 50%;
  .size(@size);
}

.font-spec(@size) {
  @base: unit(@size);
  @remValue: (@base / 10);
  font-size: ~"@{base}px";
  font-size: ~"@{remValue}rem";
}

.font-spec(@size, @weight) when (ispixel(@weight)) {
  line-height: @weight;
  .font-spec(@size);
}

.font-spec(@size, @weight) when (isnumber(@weight)) and (@weight >= 100) {
  font-weight: @weight;
  .font-spec(@size);
}

.font-spec(@size, @weight, @lineHeight) {
  line-height: @lineHeight;
  font-weight: @weight;
  .font-spec(@size);
}

.font-spec(@size, @weight, @lineHeight, @letterSpacing) {
  letter-spacing: @letterSpacing;
  line-height: @lineHeight;
  font-weight: @weight;
  .font-spec(@size);
}

styleguide.less

* {
  outline: 0;
  .box-sizing;
  &:before, &:after { .box-sizing; }
}

ol, ul {
  margin: 0; padding: 0;
  list-style-type: none;
}

a, a:visited {
  cursor: pointer;
  text-decoration: none;
  &:hover { text-decoration: none; }
}

img { max-width: 100%; height: auto; }

textarea { resize: none; }

fieldset {
  margin: 0;
  padding: 0;
  border: none;
}

.flexible {
  font-size: 0.1px;
  text-align: justify;
  -ms-text-justify: distribute-all-lines;
  text-justify: distribute-all-lines;

  > * {
    text-align: left;
    vertical-align: top;
    display: inline-block;
    float: none;
    font-size: 12px;
  }

  &:after {
    content: '';
    width: 100%;
    display: inline-block;
  }
}