Andreas Norman

This widget that displays an archive for the current selected category. On non-category pages a standard archive is displayed.
<ul>
<li>On a post page the widget will only display an archive for posts in that category</li>
<li>On a category listing the widget will only display an archive for posts in that category</li>
<li>On the front page or in a search result or any other page that doesn’t reside in a category a standard archive will be displayed.</li>
</ul>

Subzane Categorized Archive Widget

A widget that displays an archive for the current selected category. On non-category pages a standard archive is displayed.

Michael Adams (mdawaffe)

Just One Category will cause WordPress to display on a category archive page 
only those posts which are direct members of that category, i.e. this plugin 
will exclude from that category page all posts which belong to any of that 
category’s subcategories but do not belong to the category itself.

Just One Category

On a category's archive page, displays only the posts directly in that category, not in any sub-categories.

Subzane Categorized Archive Widget vs Just One Category