iaFont Awesome

fa-twitter on fa-square-o
fa-flag on fa-circle
fa-terminal on fa-square
fa-ban on fa-camera

To stack multiple icons, use the fa-stack class on the parent, the fa-stack-1x for the regularly sized icon, and fa-stack-2x for the larger icon. fa-inverse can be used as an alternative icon color. You can even throw larger icons classes on the parent to get further control of sizing.

<span class="fa-stack fa-lg">
  <i class="fa fa-square-o fa-stack-2x"></i>
  <i class="fa fa-twitter fa-stack-1x"></i>
</span>
fa-twitter on fa-square-o<br>
<span class="fa-stack fa-lg">
  <i class="fa fa-circle fa-stack-2x"></i>
  <i class="fa fa-flag fa-stack-1x fa-inverse"></i>
</span>
fa-flag on fa-circle<br>
<span class="fa-stack fa-lg">
  <i class="fa fa-square fa-stack-2x"></i>
  <i class="fa fa-terminal fa-stack-1x fa-inverse"></i>
</span>
fa-terminal on fa-square<br>
<span class="fa-stack fa-lg">
  <i class="fa fa-camera fa-stack-1x"></i>
  <i class="fa fa-ban fa-stack-2x text-danger"></i>
</span>
fa-ban on fa-camera