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.

WebshopLogic

WP Related Posts and Products plugin offers different types of similar contents for your users. For example, some related products can be displayed to your similar posts. This makes connections of your site contents visible to your clients, increasing business efficiency.
Would you like to offer some related products to your blog posts? Do you need to offer up-sell or cross-sell products? Do you have an event calendar plugin, and want to suggest some programs connected to an article? Do you have a custom movie catalog plugin and want to associate some articles to your movies?
Using cross type relationship management functions, WRI makes it possible to associate a post, page, or other custom post type to other posts, pages or custom post types, ensures widely configurable relatedness settings, sophisticated cross-relation adjustments. WRI uses the most popular YARPP relationship handling plugin in the background, extending its functionality, retaining its advantages.
WRI combines automatic, manual and common categorization based relationship management. Some functions are available in the PRO version.
You can set attributes that define how to display different type of related items for every reference item type. Such a matrix-like way you can specify all necessary variations of display settings.
This version ensures built in WooCommerce custom product support.
<h4>More information, documentation</h4>
LITE: <a href="https://webshoplogic.com/products/wordpress_plugins/wp-related-posts-and-products-lite-plugin/#tab-faq&noscroll" rel="nofollow ugc">WP Related Posts and Products Lite Documentation</a> 
PRO: <a href="https://webshoplogic.com/products/wordpress_plugins/wp-related-posts-and-products-pro-plugin/#tab-faq&noscroll" rel="nofollow ugc">WP Related Posts and Products Pro Documentation</a>
<h4>Available languages</h4>
<ul>
<li>English</li>
<li>Spanish – translated by Andrew Kurtis – WebHostingHub </li>
<li>Hungarian – translated by WebshopLogic</li>
</ul>

WP Related Posts and Products (WRI)

WP Related Items plugin offers different types of similar contents for your users. For example, some related products can be displayed to your posts.

ACF Single Relationship Add New vs WP Related Posts and Products (WRI)