Actually, in another post/thread, Susan said it best when she referred to a Snippet called IncludeFile. I use this on all my sites for my templates.
What: IncludeFile snippet. Learn about it here:
http://modxcms.com/forums/index.php/topic,4139.msg30260.html#msg30260
Why: This will allow you to add the following line in any template you create within the Manager and edit the file locally, say with Dreamweaver, which will let you edit the template "live" on your MODx site:
[[includeFile? &file=`assets/templates/templatename/index.html`]]
How: Create a Snippet called includeFile. Grab the code I used here, and paste it in the snippet:
<?php
# Snippet to include files from filesystem
# [[includeFile? &file=`assets/templates/mytemplate/file.html`]]
if ( !isset($file) || $file== "" ) return "No file specified."; //check if there's a file given.
//Start the buffer
ob_start();
//include
include $file;
//get contents from the buffer
$ob_contents = ob_get_contents();
//and kill/delete the buffer
ob_end_clean();
//return it to MODx
return $ob_contents;
?>