Lets you use conditionals for whether the page was requested via AJAX or not.
Show wrapping code if it's a regular request.
{{ unless {ajax} }}
<html>
<body>
{{ endif }}
{{ layout_content }}
{{ unless {ajax} }}
</body>
</html>
{{ endif }}
Output JSON for AJAX requests and HTML for normal requests.
There's also an {{ ajax:json_encode }} tag pair that will take care of line breaks, escaping quotes, etc.
{{ if {ajax} }}
[
{{ entries:listing }}
{
"title": {{ ajax:json_encode }}{{ title }}{{ /ajax:json_encode }}
}{{ unless last }},{{ endif }}
{{ /entries:listing }}
]
{{ else }}
<ul>
{{ entries:listing }}
<li>{{ title }}</li>
{{ /entries:listing }}
</ul>
{{ endif }}