How to add badge on top of Font Awesome symbol?

While @Paulie_D's answer is good, it doesn't work so well when you have a variable width container.

This solution works a lot better for that: http://codepen.io/johnstuif/pen/pvLgYp

HTML:

<span class="fa-stack fa-5x has-badge" data-count="8,888,888">
  <i class="fa fa-circle fa-stack-2x"></i>
  <i class="fa fa-bell fa-stack-1x fa-inverse"></i>
</span>

CSS:

.fa-stack[data-count]:after{
  position:absolute;
  right:0%;
  top:1%;
  content: attr(data-count);
  font-size:30%;
  padding:.6em;
  border-radius:999px;
  line-height:.75em;
  color: white;
  background:rgba(255,0,0,.85);
  text-align:center;
  min-width:2em;
  font-weight:bold;
}

This can be done with no additional mark-up, just a new class (which you would use anyway) and a pseudo element.

JSFiddle Demo

HTML

<i class="fa fa-envelope fa-5x fa-border icon-grey badge"></i>

CSS

*.icon-blue {color: #0088cc}
*.icon-grey {color: grey}
i {   
    width:100px;
    text-align:center;
    vertical-align:middle;
    position: relative;
}
.badge:after{
    content:"100";
    position: absolute;
    background: rgba(0,0,255,1);
    height:2rem;
    top:1rem;
    right:1.5rem;
    width:2rem;
    text-align: center;
    line-height: 2rem;;
    font-size: 1rem;
    border-radius: 50%;
    color:white;
    border:1px solid blue;
}