Thanks for the sharing this plugin.
There is one problem is this plugin.When user have mulitple post thumbnails.I kept this plugin on a page with wp-ecommerce plugin and thumbnail got messey .It was picking some random picture other than post thumbnail,I guess becausee wp-commerce and wp-ecommerc both using the_content filter.
Anyway I sorted out this issue by changing one line in post-widget.php
I thought i let you know.
I changed this line
$post_thumbnail_id = get_post_thumbnail_id( $content_post->ID);
echo $_image=wp_get_attachment_image( $post_thumbnail_id,'post-thumbnails',$attr );
( around line no 107) in
widget($args, $instance) in post-widget.php