Append before last child

This is how I got there:

http://jsfiddle.net/lharby/00tk6avg/

var htmlString = '<div class="subcontent">Some stuff</div>';

$(htmlString).insertBefore('.content div:last');

You can use insertBefore():

$('<div class="content"><div class="subcontent">Some stuff</div></div>').insertBefore('#wrapper > div:last');

Or before():

$('#wrapper > div:last').before('<div class="content"><div class="subcontent">Some stuff</div></div>');

You could use .before() to add a sibling before the element:

$("#wrapper .content:last").before('<div class="content"><div class="subcontent">Third</div></div>');

.insertBefore() does the same thing with a different syntax, namely that you select the element to be added, and pass the element you want to add it before.

$("#wrapper .content:last").before('<div class="content"><div class="subcontent">Third</div></div>');

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<div id="wrapper">
    <div class="content">
        <div class="subcontent">
            First
        </div>
    </div>
    <div class="content">
        <div class="subcontent">
            Second
        </div>
    </div>
</div>