I would not recomend you to use document.write as others suggest, because if you will open such window twice your HTML will be duplicated 2 times (or more).

Use innerHTML instead

var win = window.open("", "Title", "toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=yes,width=780,height=200,top="+(screen.height-400)+",left="+(screen.width-840));
win.document.body.innerHTML = "HTML";

Contrary to window.open(), the first (URL) argument of showModalDialog() is required. Thus, you can't pass it an HTML string. You can either use window.open:

var newWindow = window.open("", "newWindow", "resizable=yes");
newWindow.document.write('<p>Pop up window text</p>');

Or alternatively, use one the existing modal plugins, such as jQuery UI Dialog or Twitter Bootstrap Modals. They allow you to easily display a modal window based on the content of an existing HTML element, e.g.:

<div id="dialog">
  <p>Pop up window text</p>
</div>

$('#dialog').modal(); // Twitter Bootstrap
$("#dialog").dialog({ resizable: true }); // jQuery UI

Here is a simple solution that will allow you to fetch value from opened window. All you need is to inject JavaScript code into opened window that will interact with the parent window using window.opener:

HTML

<input id="value" />
<button onclick="openWindow();">Open</button>

JavaScript

function openWindow() {
    var i, l, options = [{
       value: 'first',
       text: 'First'
    }, {
       value: 'second',
       text: 'Second'
    }],
    newWindow = window.open("", null, "height=200,width=400,status=yes,toolbar=no,menubar=no,location=no");  

    newWindow.document.write("<select onchange='window.opener.setValue(this.value);'>");
    for(i=0,l=options.length; i<l; i++) {
        newWindow.document.write("<option value='"+options[i].value+"'>");  
        newWindow.document.write(options[i].text);  
        newWindow.document.write("</option>");
    }
    newWindow.document.write("</select>");
}

function setValue(value) {
    document.getElementById('value').value = value;
}

Working example here: http://jsbin.com/uqamiz/1/edit