[!DownloadBox?&thumbnail=`images/thumbnail.jpg`&downloadurl='downloads/file.zip' &description='This is the description for the download'!]
I basically want to be able to enter this into the content editor and for it to call a snippet, which calls a chunk, modifying the variables with the selected parameters:

$modx->parseChunk()
<?php
$modx->setPlaceholder(preview, $preview);
$modx->setPlaceholder(filetype, $filetype);
$modx->setPlaceholder(scalable, $scalable);
$modx->setPlaceholder(editabletext, $editabletext);
$modx->setPlaceholder(resolution, $resolution);
$modx->setPlaceholder(layered, $layered);
$modx->setPlaceholder(fonts, $fonts);
$modx->setPlaceholder(download, $download);
return $modx->getChunk('downloadbox');
?><div id="downloadimage">
<img src="[(base_url)]assets/files/downloads/[+preview+]" alt="Download Preview">
</div>
<div id="downloadcontainer">
<div id="downloadleft">
<div id="downloadwidecollumn">
<div id="row">
Format
</div>
<div id="row">
Scalable
</div>
<div id="row">
Editable Text
</div>
</div>
<div id="downloadnarrowcollumn">
<div id="row">
[+filetype+]
</div>
<div id="row">
<img src="[(base_url)]assets/templates/stemplate/images/[+scalable+].png" alt="scalable">
</div>
<div id="row">
<img src="[(base_url)]assets/templates/stemplate/images/[+editabletext+].png" alt="editable text"> </div>
</div>
<div id="downloadwidecollumn">
<div id="row">
Resolution
</div>
<div id="row">
Layered
</div>
<div id="row">
Fonts
</div>
</div>
<div id="downloadnarrowcollumn">
<div id="row">[+resolution+]
</div>
<div id="row">
<img src="[(base_url)]assets/templates/stemplate/images/[+layered+].png" alt="layered">
</div>
<div id="row">[+fonts+]</div>
</div>
</div>
<div id="downloadright">
<a href="[(base_url)]assets/files/downloads/[+download+]"><img src="[(base_url)]assets/templates/stemplate/images/downloadbutton.png" alt="Download file"></a> </div>
</div>[[downloadbox? &preview=banners/banner1.jpg&filetype=.PSD&scalable=tick&editabletext=tick&resolution=300x500&layered=tick&fonts=Arial&download=banners/banner1.psd]]
I found that printing out the document.parser.inc.php file and using it for evening reading was very useful. It took a few passes, but I finally began to get a good understanding of what what going on under the hood.
<h1>My Big Listing</h1> <ul> [makelist] </ul>
$output = array (
$preview => 'preview',
$filetype => 'filetype',
$scalable => 'scalable',
$editabletext => 'editabletext',
$resolution => 'resolution',
$layered => 'layered',
$fonts => 'fonts',
$download => 'download'
);
$output .= $modx->parseChunk('downloadbox', $array);