Stephen Mekosh

Ordered Thumbnails is a WordPress plugin that allows you to display thumbnails based on images that have been attached to a post. This plugin also allows you to specify which thumbnail appears next to a post with multiple images by ordering the thumbnails in the Gallery tab of the WordPress “Add Images” lightbox and placing the thumbnail desired in the first position.
To style how the thumbnail appears on your page, you may pass optional CSS class names to the plugin, which will then be applied to the <code><img></code> tag returned. Alternately, by setting the display parameter to “false”, Ordered Thumbnails will return the thumbnail URL, height, and width as a PHP array to be used however you see fit. If no images have been attached to a post, the plugin fails gracefully and returns “false”. This plugin will work anywhere the WordPress posts loop is used and works particularly nice in combination with <code>the_excerpt</code> for archive pages.

Ordered Thumbnails

Display a thumbnail based on post images with no additional uploads. For multiple image posts, thumbnail respects Gallery sort order.

Dave Ligthart

A simple way to choose a default thumbnail for your post.
Works alongside the featured image core functionality.
For theme usage:
<h4>Add this function where you want the thumbnail to appear</h4>
<pre><code><?php if(function_exists('wct_thumb')){ wct_thumb(); } ?>
</code></pre>

WP-Choose-Thumb

A simple way to add a default thumbnail to your post.

Ordered Thumbnails vs WP-Choose-Thumb