<div id='myDiv'></div>

$('#myDiv').html("<div id='mySecondDiv'></div>");

var mySecondDiv=$('<div id="mySecondDiv"></div>');
$('#myDiv').append(mySecondDiv);

jQuery(Array(101).join('<div></div>'));

$('<div id="' + someID + '" class="foobar">' + content + '</div>');

$('<div/>', {
    id: someID,
    className: 'foobar',
    html: content
});

$("#myDiv").html('<div id="mySecondDiv"></div>');

$("#myDiv").append('<div id="mySecondDiv"></div>');

var mySecondDiv=$('<div id="mySecondDiv"></div>');
$('#myDiv').html(mySecondDiv);

var mySecondDiv=$('<div id="mySecondDiv"></div>');
$('#myDiv').append(mySecondDiv);

var mySecondDiv = $("<div id='mySecondDiv'></div>").appendTo("#myDiv");
// other code here
mySecondDiv.hide();

var mySecondDiv = $('<div></div>');
$(mySecondDiv).find('div').attr('id', 'mySecondDiv');
$('#myDiv').append(mySecondDiv);

// Clumsy string concat, error prone
$('#myDiv').html("<div style='width:'" + myWidth + "'px'>Lorem ipsum</div>");

// Isn't this a lot cleaner? (though longer)
var newDiv = $('<div></div>');
$(newDiv).find('div').css('width', myWidth);
$('#myDiv').append(newDiv);

<div id="test">
    <input type="file" name="file0" onchange="changed()">
</div>
<script type="text/javascript">
    var isAllowed = true;
    function changed()
    {
        if (isAllowed)
        {
            var tmpHTML = $('#test').html();
            tmpHTML += "<input type="file" name="file1" onchange="changed()">";
            $('#test').html(tmpHTML);
            isAllowed = false;
        }
    }
</script>

function changed()
    {
        if (isAllowed)
        {
            var tmpHTML = "<input type="file" name="file1" onchange="changed()">";
            $('#test').append(tmpHTML);
            isAllowed = false;
        }
    }