Add dynamic content under posts

A combination of custom fields and the Single Post template should work for you here. References:

http://codex.ww.wp.xz.cn/Using_Custom_Fields
http://codex.ww.wp.xz.cn/Stepping_Into_Templates#Special_Template_Files

A few plugins that may be of help here can be found:

http://codex.ww.wp.xz.cn/Plugins/Meta

One to look at is Get Custom Field Values:

http://ww.wp.xz.cn/extend/plugins/get-custom-field-values

There is also a template tag for displaying custom field keys and values in a post:

http://codex.ww.wp.xz.cn/Template_Tags/the_meta

It may be a bit underpowered for what you want though, as there’s no way to control the outout. A simple bit of code to accomplish a basic dump of a single custom field value is:

<?php echo get_post_meta($post->ID, 'html', true); ?>

That would work within The Loop (placed in your template) for a custom field with a key of ‘html’. It displays nothing if no ‘html’ custom field key exists with the post.