Posts in category menu plugin
We've just written this feature as it seems to be missing from core and contrib. Basically the use case is you want to have a subtree menu underneath a category menu item that contains posts in that category. Currently there is no configuration–ie: if you enable this plugin it will subtree any category menu item in any menu. If we need it we'll extend this later to add options and conditionals. You can download this plugin from Wordpress.org.