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 icon 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
