fancyfiber

Requires the plug-in <a href="https://wordpress.org/extend/plugins/advanced-custom-fields/" title="Awesome plug-in to manage custom fields" rel="ugc">Advanced Custom Fields</a>.
This is a very basic add-on to allow the ability for relationship fields that are associated with a single related item, to be able to add a new 
item to the related item without leaving the post/page you are currently editing. Uses a colorbox pop-up to open the add new post window with very 
limited options.
Usage:
<ul>
<li>Install the plug-in (standard WordPress installation)</li>
<li>Set up a relationship field with only one related item (standard Advanced Custom Field configuration)</li>
<li>Add or Edit a post with the relationship and the new Add New button will show up on the relationship field</li>
<li>Click the Add New button to open Colorbox add new post for the related item</li>
<li>Click the publish button and the new item will be added (or click the X button to cancel) and the Colorbox will close</li>
<li>The newly added item will be available for selection in the relationship list</li>
</ul>
This plug-in was developed for <a href="http://www.tanabi.com" rel="nofollow ugc">Tanabicom, LLC</a> and <a href="http://www.5dspectrum.com" rel="nofollow ugc">5D Spectrum, Inc</a> by <a href="http://hosting.fancyfiber.com/design/" rel="nofollow ugc">Fancy Fiber Designs</a>

ACF Single Relationship Add New

Add a new related item while editing a post, without leaving the current post.

bachiller

If you have a bounce rate problem or your readers don’t stay too long on your web, Relate-it can help. It shows thumbnails of related posts as an alternative to link lists or tags.
The thumbnails will appear between the content of the post and the comments section.
The relationship between the posts is established based on two criteria: same tag or category. First the plugin looks for posts wiht the same tags as the main post. If the number of obtained posts doesn’t reach a value specified in the plugin settings section then it looks for posts in the same category. Finally, to complete the number of related posts, it uses random posts.
** Features ** * Selection of related posts based on tags and categories criteria. * Thumbnail with the images of selected posts shown just below the main post content. * Links to selected posts. * Independent CSS stylesheet to change easly colors and visual styles. * Configurable number of related posts. * Configurable caption to change the text the plugin shows.
** Administration ** Relate-it plugin has some settings that an admin can manage easily with a form in the settings section. These are the options that can be configured: * Caption: A text that will be shown above the thumnail. * Maximun lenght of related post title. * Number of related posts that wil be shown.
** Appearance ** Relate-it plugin includes a stylesheet css file that can be easily modified to change the appearance of the thumbnail. All the styles used by the plugin can be found in the following file: /wp-content/plugins/relate-it/css/style.css

Relate-it

Relate-it is a very simple plugin that shows thumbnails of posts related to the blog entry, based on the same tag or category.

ACF Single Relationship Add New vs Relate-it